Surface acoustic wave device

ABSTRACT

A surface including interdigital electrode on the surface thereof, which is reduced in size and improved in selectivity, and has a broad band. To achieve this, a langasite single crystal belonging to a point group 32 is first used for the substrate. Secondly, I. a piezoelectric film is provided for covering the surface of the substrate and the surface of the interdigital electrode, II. a piezoelectric film is provided on the surface of the substrate and the interdigital electrode is provided on the surface of the piezoelectric film, III. a piezoelectric film is provided for covering the surface of the substrate and the surface of the interdigital electrode and an opposite electrode film is provided on the surface of the piezoelectric film, or IV. an opposite electrode film is provided on the surface of the substrate, a piezoelectric film is provided on the opposite electrode film and the interdigital electrode is provided on the surface of the piezoelectric film. The piezoelectric film is made up of zinc oxide, and has a piezoelectric axis oriented substantially perpendicularly with respect to the surface of the substrate. The cut angle of the substrate cut out of the langasite single crystal, the propagation direction of surface acoustic waves on the substrate, and the thickness of the piezoelectric film are optimized depending on how to provide the piezoelectric film.

BACKGROUND OF THE INVENTION

1. Field of the Invention

The present invention relates to a surface acoustic wave devicecomprising an interdigital electrode on a single crystal substrate.

2. Discussion of the Background

In recent years, mobile communication terminal equipment inclusive ofcellular telephones has been rapidly popularized. This terminalequipment is particularly desired to be reduced in size and weight forreason of portability. To achieve size and weight reductions for theterminal equipment, electronic parts used therewith, too, should beessentially reduced in size and weight. For this reason, surfaceacoustic wave devices favorable for size and weight reductions, i.e.,surface acoustic wave filters are often used for high- andintermediate-frequency parts of the terminal equipment. A surfaceacoustic wave device has on the surface of a piezoelectric substrate aninterdigital electrode for exciting, receiving, reflecting, andpropagating surface acoustic waves.

Among characteristics important to a piezoelectric substrate used forsurface acoustic wave devices, there are the surface wave velocity ofsurface acoustic waves (SAW velocity), the temperature coefficient of acenter frequency in the case of filters and of a resonance frequency inthe case of resonators (the temperature coefficient of frequency: TCF),and an electromechanical coupling factor (k²). Set out in Table 1 arethe characteristics of various piezoelectric substrate known so far forsurface acoustic wave devices. Hereinafter, these piezoelectricsubstrates will be referred to by the symbols used in Table 1. In thisregard, it is to be noted that TCV (the temperature coefficient of SAWvelocity) is a quantity representing the temperature dependence of thevelocity of surface acoustic waves (the SAW velocity); that is, it has avalue equivalent to that of the aforesaid TCF representing thetemperature dependence of the center or resonance frequency. A large TCVvalue implies that the center frequency of a surface acoustic wavefilter varies significantly with temperature.

                                      TABLE 1                                     __________________________________________________________________________                       Propagation                                                                           SAW Velocity                                                                         k.sup.2                                                                          TCV                                      Symbol                                                                                Composition                                                                       Cut Angle                                                                                Direction                                                                              (m/s)                                                                                (ppm/° C.)                      __________________________________________________________________________    128LN                                                                              LiNbO.sub.3                                                                         128°-Rotation Y                                                                X       3992   5.5                                                                              -74                                      64LN           64°-Rotation Y                                                                 X               -79.3                                  LT112                                                                                                     3288degree.-Rotation Y                                                                 0.64                                                                                -18                                36LT           36°-Rotation Y                                                                 X               -45.7                                  ST Crystal                                                                           Quartz                                                                                                        0 (primary coef.)                      BGO              (100) GeO.sub.20                                                                     (011)                                                                                        -1222                                  __________________________________________________________________________

As can be seen from Table 1, 64LN and 36LT have an SAW velocity of 4,000m/s or higher, and so are suitable to construct filters forhigh-frequency parts of terminal equipment. Referring now to the reasonfor this, various systems are practically employed for mobilecommunications represented by cellular telephones all over the world,and are all used at frequencies of the order of 1 GHz. Accordingly,filters used for high-frequency parts of terminal equipment have acenter frequency of approximately 1 GHz. Surface acoustic wave filtershave a center frequency substantially proportional to the SAW velocitiesof piezoelectric substrates used but almost inversely proportional tothe widths of electrode fingers formed on Ad substrates. To enable suchfilters to be operated at high frequencies, therefore, it is preferableto resort to substrates having high SAW velocities, for instance, 64LN,and 36LT. Also, wide passband widths of 20 MHz or more are required forfilters used on high-frequency parts. To achieve such broad passbands,however, it is essentially required that piezoelectric substrates have alarge electromechanical coupling factor k². For these reasons, much useis made of 64LN, and 36LT.

On the other hand, a frequency band of 70 to 300 MHz is used as anintermediate frequency for mobile terminal equipment. When a filterusing this frequency band as a center frequency is constructed with theuse of a surface acoustic wave device, the use of the aforesaid 64LN or36LT as a piezoelectric substrate causes the width of an electrodefinger formed on the substrate to be much larger than that of theaforesaid filter used for a high-frequency part.

This will now be explained with reference to roughly calculated specificvalues. Here let d represent the width of an electrode finger of asurface acoustic wave transducer that forms a surface acoustic wavefilter, f₀ indicate the center frequency of the surface acoustic wavefilter, and V denote the SAW velocity of the piezoelectric substrateused. For these values, equation (1) then holds roughly

    f.sub.0 =V/(4d)                                            (1)

If a surface acoustic wave filter having a center frequency of 1 GHz isconstructed on the assumption that the SAW velocity is 4,000 m/s, thenthe width of its electrode finger is calculated from equation (1) to be

    d=4,000 (m/s)/[(4×1,000 (MHz)]=1 μm

on the other hand, when an intermediate-frequency filter having a centerfrequency of 100 MHz is constructed using this piezoelectric substratehaving an SAW velocity of 4,000 m/s, the electrode finger width requiredfor this is given by

    d=4,000 (m/s)/[(4×100 (MHz)]=10 μm

Thus, the required electrode finger width is 10 times as large as thatfor the high-frequency part filter. A large electrode finger widthimplies that a surface acoustic wave device itself becomes large. Tomake a surface acoustic wave intermediate-frequency filter small,therefore, it is necessary to use a piezoelectric substrate having a lowSAW velocity V, as can be appreciated from the aforesaid equation (1).

Among piezoelectric substrates known to have very limited SAW velocity,there is BGO such as one already referred to in the aforesaid Table 1. ABGO piezoelectric substrate has an SAW velocity of 1,681 m/s. However,the BGO piezoelectric substrate is unsuitable to construct anintermediate- frequency filter for extracting one channel signal alone,because its temperature coefficient of SAW velocity or its TCV is aslarge as -122 ppm/° C. The reason is that TCV is the quantity indicativeof the temperature dependence of SAW velocity as already noted, and thata large TCV value implies that the center frequency of the surfaceacoustic wave filter varies largely with temperature, as can again beseen from the aforesaid equation (1). Thus, large TCV is unsuitable foran intermediate-frequency filter because undesired signals may possiblybe extracted from other channel adjacent to the desired channel.

Among piezoelectric substrates known to have relatively low SAW velocitythere is ST quartz crystal such as one referred to in the aforesaidTable 1. The ST quartz crystal is suitable to construct anintermediate-frequency filter because its temperature coefficient of SAWvelocity or its TCV is almost zero (with a primary temperaturecoefficient a of zero). For this reason, most of intermediate-frequencysurface acoustic wave filters used so far for mobile communicationterminal equipment are constructed of ST quartz crystal piezoelectricsubstrates.

However, the SAW velocity of the ST quartz crystal substrate is 3,158m/s or is not on a sufficiently reduced level, and so presents somelimitation on size reductions.

In addition, the electromechanical coupling factor k² of the ST quartzcrystal is 0.14%, and so is relatively small. Small k² implies that onlya filter having a narrow passband is achievable. Adopted mainly so farfor mobile communications, that is, cellular telephones are analogsystems with a very narrow channel band width of, for instance, 12.5 kHzaccording to the Japanese NTT standard, 30 kHz according to the U.S.AMPS standard, and 25 kHz according to the European TACS standard. Thus,the fact that the aforesaid ST quartz crystal has a smallelectromechanical coupling factor k² has offered no problem whatsoever.In recent years, however, digital mobile communication systems have beendeveloped, put to practical use, and so rapidly widespread in view ofmaking effective use of frequency resources, compatibility with digitaldata communications, etc. The channel width of this digital system isvery wide, for instance, 200 kHz and 1.7 MHz in the European cellulartelephone GSM and cordless telephone DECT modes, respectively. If STquartz crystal substrates are used for surface acoustic wave filters, itis then difficult to construct such wide-band intermediate-frequencyfilters using them.

On the other hand, it is known that the electromechanical couplingfactor of a surface acoustic surface device can be increased by forminga piezoelectric film made up of zinc oxide, tantalum oxide, CdS or thelike on the surface of a piezoelectric substrate made up of LiNbO₃ orthe like, as typically set forth in JP-A 8-204499. However, aconventional piezoelectric substrate such as an LiNbO₃ substrate is notpreferable because its temperature coefficient of SAW velocity, TCV, isnegative, and so its overall TCV is greatly shifted to a negative sidewhen a zinc oxide film is provided thereon.

As explained above, a problem with a conventional surface acoustic wavedevice is that when a piezoelectric substrate such as the aforesaid64LN, 36LT or the like is used, it is possible to make its passbandbroad, but device size becomes large because that substrate has high SAWvelocity. Another problem is that when the aforesaid BGO substratehaving low SAW velocity is used to achieve device size reductions,good-enough selectivity is not obtained due to too large a temperaturecoefficient of SAW velocity or TCV. In either case, characteristics goodenough for any intermediate- frequency surface acoustic wave filter areunachievable.

The ST quartz crystal substrate having a small temperature coefficientof SAW velocity, TCV, presents some limitation on size reductions due tothe fact that its SAW velocity is not sufficiently reduced, and makes itdifficult to achieve wide band due to the fact that its electro-mechanical coupling factor is relatively small.

SUMMARY OF THE INVENTION

It is an object of the invention to provide a surface acoustic wavedevice which is of small size, and good-enough selectivity. Anotherobject of the invention is to provide a surface acoustic wave devicewhich is of small size, and broad passband. Yet another object of theinvention is to provide a surface acoustic wave device which is of smallsize, and good-enough selectivity, and broad passband.

The aforesaid objects are achievable by any one of the embodimentsrecited below as 1 to 4.

Embodiment 1

(1) A surface acoustic wave device comprising a substrate, aninterdigital electrode on a surface thereof, and a piezoelectric filmfor covering said surface of said substrate and a surface of saidinterdigital electrode, wherein:

said substrate is a langasite single crystal belonging to a point group32, and said piezoelectric film is made up of zinc oxide.

(2) The surface acoustic wave device of (1), wherein said piezoelectricfilm has a piezoelectric axis oriented substantially perpendicularlywith respect to said surface of said substrate.

(3) The surface acoustic wave device of (1), wherein:

when a cut angle of said substrate cut out of the langasite singlecrystal and a propagation direction of a surface acoustic wave on saidsubstrate are represented in terms of Euler angles (φ, θ, ψ), φ, θ, andψ lie within the following general area I:

Area I

-5°≦φ≦ to 5°

85°≦θ≦95°

-90°≦ψ<90°

(4) The surface acoustic wave device of (1), wherein:

when a cut angle of said substrate cut out of the langasite singlecrystal and a propagation direction of a surface acoustic wave on saidsubstrate are represented in terms of Euler angles (φ, θ, ψ), φ, θ, andψ lie within the following area I-1:

Area I-1

-5°≦φ≦5°

85°≦θ≦95°

-90°≦ψ<-70°

(5) The surface acoustic wave device of (4), which satisfies:

    h/λ=0.2 to 0.8

where h is a thickness of said piezoelectric film on said surface ofsaid substrate, and λ is a wavelength of said surface acoustic wave.

(6) The surface acoustic wave device of (1), wherein:

when a cut angle of said substrate cut out of the langasite singlecrystal and a propagation direction of a surface acoustic wave on saidsubstrate are represented in terms of Euler angles (φ, θ, ψ), φ, θ, andψ lie within the following area I-2:

Area I-2

-5°≦φ≦5°

85°≦θ≦95°

-70°ψ<-50°

(7) The surface acoustic wave device of (6), which satisfies:

    h/λ=0.25 to 0.7

where h is a thickness of said piezoelectric film on said surface ofsaid substrate, and λ is a wavelength of said surface acoustic wave.

(8) The surface acoustic wave device of (1), wherein:

when a cut angle of said substrate cut out of the langasite singlecrystal and a propagation direction of a surface acoustic wave on saidsubstrate are represented in terms of Euler angles (φ, θ, ψ), φ, θ, andψ lie within the following area I-3:

Area I-3

-5°≦φ≦5°

85°≦θ≦95°

-50°≦ψ<-35°

(9) The surface acoustic wave device of (8), which satisfies:

    h/λ=0.25 to 0.45

where h is a thickness of said piezoelectric film on said surface ofsaid substrate, and λ is a wavelength of said surface acoustic wave.

(10) The surface acoustic wave device of (1), wherein:

when a cut angle of said substrate cut out of the langasite singlecrystal and a propagation direction of a surface acoustic wave on saidsubstrate are represented in terms of Euler angles (φ, θ, ψ), φ, θ, andψ lie within the following area I-4:

Area I-4

-5°≦φ≦5°

85°≦θ≦95°

-35°≦ψ<-25°

(11) The surface acoustic wave device of (10), which satisfies:

    0<h/λ≦0.5

where h is a thickness of said piezoelectric film on said surface ofsaid substrate, and λ is a wavelength of said surface acoustic wave.

(12) The surface acoustic wave device of (1), wherein:

when a cut angle of said substrate cut out of the langasite singlecrystal and a propagation direction of a surface acoustic wave on saidsubstrate are represented in terms of Euler angles (φ, θ, ψ), φ, θ, andψ lie within the following area I-5:

Area I-5

-5°≦φ≦5°

85°≦θ≦95°

-25°≦ψ≦-10°

(13) The surface acoustic wave device of (12), which satisfies:

    0<h/λ≦0.45

where h is a thickness of said piezoelectric film on said surface ofsaid substrate, and λ is a wavelength of said surface acoustic wave.

(14) The surface acoustic wave device of (1), wherein:

when a cut angle of said substrate cut out of the langasite singlecrystal and a propagation direction of a surface acoustic wave on saidsubstrate are represented in terms of Euler angles (φ, θ, ψ), φ, θ, andψ lie within the following area I-6:

Area I-6

-5°≦φ≦5°

85°≦θ≦95°

10°≦ψ<25°

(15) The surface acoustic wave device of (14), which satisfies:

    0<h/λ≦0.4

where h is a thickness of said piezoelectric film on said surface ofsaid substrate, and k is a wavelength of said surface acoustic wave.

(16) The surface acoustic wave device of (1), wherein:

when a cut angle of said substrate cut out of the langasite singlecrystal and a propagation direction of a surface acoustic wave on saidsubstrate are represented in terms of Euler angles (φ, θ, ψ), φ, θ, andψ lie within the following area I-7:

Area I-7

-5°≦φ≦5°

85°≦θ≦95°

25°≦ψ<35°

(17) The surface acoustic wave device of (16), which satisfies:

    0<h/λ≦0.45

where h is a thickness of said piezoelectric film on said surface ofsaid substrate, and λ is a wavelength of said surface acoustic wave.

(18) The surface acoustic wave device of (1), wherein:

when a cut angle of said substrate cut out of the langasite singlecrystal and a propagation direction of a surface acoustic wave on saidsubstrate are represented in terms of Euler angles (φ, θ, ψ), φ, θ, andψ lie within the following area I-8:

Area I-8

-5°≦φ≦5°

85°≦θ≦95°

35°≦ψ<50°

(19) The surface acoustic wave device of (18), which satisfies:

    0<h/λ≦0.4

where h is a thickness of said piezoelectric film on said surface ofsaid substrate, and λ is a wavelength of said surface acoustic wave.

(20) The surface acoustic wave device of (1), wherein:

when a cut angle of said substrate cut out of the langasite singlecrystal and a propagation direction of a surface acoustic wave on saidsubstrate are represented in terms of Euler angles (φ, θ, ψ), φ, θ, andψ lie within the following area I-9:

Area I-9

-5°≦φ≦5°

85°≦θ≦95°

50°≦ψ<70°

(21) The surface acoustic wave device of (20), which satisfies:

    h/λ=0.15 to 0.7

where h is a thickness of said piezoelectric film on said surface ofsaid substrate, and λ is a wavelength of said surface acoustic wave.

(22) The surface acoustic wave device of (1), wherein:

when a cut angle of said substrate cut out of the langasite singlecrystal and a propagation direction of a surface acoustic wave on saidsubstrate are represented in terms of Euler angles (φ, θ, ψ), φ, θ, andψ lie within the following area I-10:

Area I-10

-5°≦φ≦5°

85°≦θ≦95°

70°≦ψ<90°

(23) The surface acoustic wave device of (22), which satisfies:

    h/λ=0.15 to 0.8

where h is a thickness of said piezoelectric film on said surface ofsaid substrate, and λ is a wavelength of said surface acoustic wave.

Embodiment 2

(1) A surface acoustic wave device comprising a substrate, apiezoelectric film on a surface thereof, and an interdigital electrodeon a surface of said piezoelectric film, wherein:

said substrate is a langasite single crystal belonging to a point group32, and said piezoelectric film is made up of zinc oxide.

(2) The surface acoustic wave device of (1), wherein said piezoelectricfilm has a piezoelectric axis oriented substantially perpendicularlywith respect to said surface of said substrate.

(3) The surface acoustic wave device of (1), wherein:

when a cut angle of said substrate cut out of the langasite singlecrystal and a propagation direction of a surface acoustic wave on saidsubstrate are represented in terms of Euler angles (φ, θ, ψ), φ, θ, andψ lie within the following general area II:

Area II

-5°≦φ≦5°

85°≦θ≦95°

-90°≦ψ<90°

(4) The surface acoustic wave device of (1), wherein:

when a cut angle of said substrate cut out of the langasite singlecrystal and a propagation direction of a surface acoustic wave on saidsubstrate are represented in terms of Euler angles (φ, θ, ψ), φ, θ, andψ lie within the following area II-1:

Area II-1

-5°≦φ≦5°

85°≦θ≦95°

-90°≦ψ<-70°

(5) The surface acoustic wave device of (4), which satisfies:

    h/λ=0.05 to 0.8

where h is a thickness of said piezoelectric film, and λ is a wavelengthof said surface acoustic wave.

(6) The surface acoustic wave device of (1), wherein:

when a cut angle of said substrate cut out of the langasite singlecrystal and a propagation direction of a surface acoustic wave on saidsubstrate are represented in terms of Euler angles (φ, θ, ψ), φ, θ, andψ lie within the following area II-2:

Area II-2

-5°≦φ≦5°

85°≦θ≦95°

-70°≦ψ<-50°

(7) The surface acoustic wave device of (6), which satisfies:

    h/λ=0.05 to 0.75

where h is a thickness of said piezoelectric film, and λ is a wavelengthof said surface acoustic wave.

(8) The surface acoustic wave device of (1), wherein:

when a cut angle of said substrate cut out of the langasite singlecrystal and a propagation direction of a surface acoustic wave on saidsubstrate are represented in terms of Euler angles (φ, θ, ψ), φ, θ, andψ lie within the following area II-3:

Area II-3

-5°≦φ≦5°

85°≦θ≦95°

-50°≦ψ<-35°

(9) The surface acoustic wave device of (8), which satisfies:

    0<h/λ≦0.45

where h is a thickness of said piezoelectric film, and λ is a wavelengthof said surface acoustic wave.

(10) The surface acoustic wave device of (1), wherein:

when a cut angle of said substrate cut out of the langasite singlecrystal and a propagation direction of a surface acoustic wave on saidsubstrate are represented in terms of Euler angles (φ, θ, ψ), φ, θ, andψ lie within the following area II-4:

Area II-4

-5°≦φ≦5°

85°≦θ≦95°

-35°≦ψ<-25°

(11) The surface acoustic wave device of (10), which satisfies:

    0<h/λ≦0.5

where h is a thickness of said piezoelectric film, and λ is a wavelengthof said surface acoustic wave.

(12) The surface acoustic wave device of (1), wherein:

when a cut angle of said substrate cut out of the langasite singlecrystal and a propagation direction of a surface acoustic wave on saidsubstrate are represented in terms of Euler angles (φ, θ, ψ), φ, θ, andψ lie within the following area II-5:

Area II-5

-5°≦φ≦5°

85°≦θ≦95°

-25°≦ψ<-10°

(13) The surface acoustic wave device of (12), which satisfies:

    0<h/λ≦0.45

where h is a thickness of said piezoelectric film, and λ is a wavelengthof said surface acoustic wave.

(14) The surface acoustic wave device of (1), wherein:

when a cut angle of said substrate cut out of the langasite singlecrystal and a propagation direction of a surface acoustic wave on saidsubstrate are represented in terms of Euler angles (φ, θ, ψ), φ, θ, andψ lie within the following area II-6:

Area II-6

-5°≦φ≦5°

85°≦θ≦95°

10°≦ψ<25°

(15) The surface acoustic wave device of (14), which satisfies:

    0<h/λ≦0.4

where h is a thickness of said piezoelectric film on said surface ofsaid substrate, and λ is a wavelength of said surface acoustic wave.

(16) The surface acoustic wave device of (1), wherein:

when a cut angle of said substrate cut out of the langasite singlecrystal and a propagation direction of a surface acoustic wave on saidsubstrate are represented in terms of Euler angles (φ, θ, ψ), φ, θ, andψ lie within the following area II-7:

Area II-7

-5°≦φ≦5°

85°≦θ≦95°

25°≦ψ<35°

(17) The surface acoustic wave device of (16), which satisfies:

    0<h/λ≦0.45

where h is a thickness of said piezoelectric film, and λ is a wavelengthof said surface acoustic wave.

(18) The surface acoustic wave device of (1), wherein:

when a cut angle of said substrate cut out of the langasite singlecrystal and a propagation direction of a surface acoustic wave on saidsubstrate are represented in terms of Euler angles (φ, θ, ψ), φ, θ, andψ lie within the following area II-8:

Area II-8

-5°≦φ≦5°

85°≦θ≦95°

35°≦ψ<50°

(19) The surface acoustic wave device of (18), which satisfies:

    0<h/λ<0.4

where h is a thickness of said piezoelectric film, and λ is a wavelengthof said surface acoustic wave.

(20) The surface acoustic wave device of (1), wherein:

when a cut angle of said substrate cut out of the langasite singlecrystal and a propagation direction of a surface acoustic wave on saidsubstrate are represented in terms of Euler angles (φ, θ, ψ), φ, θ, andψ lie within the following area II-9:

Area II-9

-5°≦φ≦5°

85°≦θ≦95°

50°≦ψ<70°

(21) The surface acoustic wave device of (20), which satisfies:

    h/λ=0.05 to 0.7

where h is a thickness of said piezoelectric film, and λ is a wavelengthof said surface acoustic wave.

(22) The surface acoustic wave device of (1), wherein:

when a cut angle of said substrate cut out of the langasite singlecrystal and a propagation direction of a surface acoustic wave on saidsubstrate are represented in terms of Euler angles (φ, θ, ψ), φ, θ, andψ lie within the following area II-10:

Area II-10

-5°≦φ≦5°

85°≦θ≦95°

70°≦ψ<90°

(23) The surface acoustic wave device of (22), which satisfies:

    h/λ=0.05 to 0.8

where h is a thickness of said piezoelectric film, and λ is a wavelengthof said surface acoustic wave.

Embodiment 3

(1) A surface acoustic wave device comprising a substrate, aninterdigital electrode on a surface thereof, a piezoelectric film forcovering said surface of said substrate and a surface of saidinterdigital electrode, and an opposite electrode film on a surface ofsaid piezoelectric film, wherein:

said substrate is a langasite single crystal belonging to a point group32, and said piezoelectric film is made up of zinc oxide.

(2) The surface acoustic wave device of (1), wherein said piezoelectricfilm has a piezoelectric axis oriented substantially perpendicularlywith respect to said surface of said substrate.

(3) The surface acoustic wave device of (1), wherein:

when a cut angle of said substrate cut out of the langasite singlecrystal and a propagation direction of a surface acoustic wave on saidsubstrate are represented in terms of Euler angles (φ, θ, ψ), φ, θ, andψ lie within the following general area III:

Area III

-5°≦φ≦5°

85°≦θ≦95°

90°≦ψ<90°

(4) The surface acoustic wave device of (1), wherein:

when a cut angle of said substrate cut out of the langasite singlecrystal and a propagation direction of a surface acoustic wave on saidsubstrate are represented in terms of Euler angles (φ, θ, ψ), φ, θ, andψ lie within the following area III-1:

Area III-1

-5°≦φ≦5°

85°≦θ≦95°

90°≦ψ<-70°

(5) The surface acoustic wave device of (4), which satisfies:

    0<h/λ≦0.1

where h is a thickness of said piezoelectric film on said surface ofsaid substrate, and λ is a wavelength of said surface acoustic wave.

(6) The surface acoustic wave device of (4), which satisfies:

    h/λ=0.3 to 0.8

where h is a thickness of said piezoelectric film on said surface ofsaid substrate, and λ is a wavelength of said surface acoustic wave.

(7) The surface acoustic wave device of (1), wherein:

when a cut angle of said substrate cut out of the langasite singlecrystal and a propagation direction of a surface acoustic wave on saidsubstrate are represented in terms of Euler angles (φ, θ, ψ), φ, θ, andψ lie within the following area III-2:

Area III-2

-5°≦φ≦5°

85°≦θ≦95°

70°≦ψ<-50°

(8) The surface acoustic wave device of (7), which satisfies:

    0<h/λ≦0.1

where h is a thickness of said piezoelectric film on said surface ofsaid substrate, and λ is a wavelength of said surface acoustic wave.

(9) The surface acoustic wave device of (7), which satisfies:

    h/λ=0.35 to 0.8

where h is a thickness of said piezoelectric film on said surface ofsaid substrate, and λ is a wavelength of said surface acoustic wave.

(10) The surface acoustic wave device of (1), wherein:

when a cut angle of said substrate cut out of the langasite singlecrystal and a propagation direction of a surface acoustic wave on saidsubstrate are represented in terms of Euler angles (φ, θ, ψ), φ, θ, andψ lie within the following area III-3:

Area III-3

-5°≦φ≦5°

85°≦θ≦95°

-50°≦ψ≦-35° from which -30° is excluded

(11) The surface acoustic wave device of (10), which satisfies:

    0<h/λ≦0.15

where h is a thickness of said piezoelectric film on said surface ofsaid substrate, and λ is a wavelength of said surface acoustic wave.

(12) The surface acoustic wave device of (10), which satisfies:

    h/λ=0.35 to 0.5

where h is a thickness of said piezoelectric film on said surface ofsaid substrate, and λ is a wavelength of said surface acoustic wave.

(13) The surface acoustic wave device of (1), wherein:

when a cut angle of said substrate cut out of the langasite singlecrystal and a propagation direction of a surface acoustic wave on saidsubstrate are represented in terms of Euler angles (, θ, ψ), φ, θ, and ψlie within the following area III-4:

Area III-4

-5°≦φ≦5°

85°≦θ≦95°

-35°≦ψ≦-25°

(14) The surface acoustic wave device of (13), which satisfies:

    0<h/λ≦0.15

where h is a thickness of said piezoelectric film on said surface ofsaid substrate, and λ is a wavelength of said surface acoustic wave.

(15) The surface acoustic wave device of (13), which satisfies:

    h/λ=0.3 to 0.5

where h is a thickness of said piezoelectric film on said surface ofsaid substrate, and λ is a wavelength of said surface acoustic wave.

(16) The surface acoustic wave device of (1), wherein:

when a cut angle of said substrate cut out of the langasite singlecrystal and a propagation direction of a surface acoustic wave on saidsubstrate are represented in terms of Euler angles (φ, θ, ψ), φ, θ, andψ lie within the following area III-5:

Area III-5

-5°≦φ≦5°

85°≦θ≦95°

-25°≦ψ≦-10°

(17) The surface acoustic wave device of (16), which satisfies:

    0<h/λ≦0.15

where h is a thickness of said piezoelectric film on said surface ofsaid substrate, and λ is a wavelength of said surface acoustic wave.(18) The surface acoustic wave device of (16), which satisfies:

    h/λ=0.3 to 0.45

where h is a thickness of said piezoelectric film on said surface ofsaid substrate, and λ is a wavelength of said surface acoustic wave.

(19) The surface acoustic wave device of (1), wherein:

when a cut angle of said substrate cut out of the langasite singlecrystal and a propagation direction of a surface acoustic wave on saidsubstrate are represented in terms of Euler angles (φ, θ, ψ), φ, θ, andψ lie within the following area III-6:

Area III-6

-5°≦φ≦5°

85°≦θ≦95°

10°≦ψ<25°

(20) The surface acoustic wave device of (19), which satisfies:

    0<h/λ≦0.45

where h is a thickness of said piezoelectric film on said surface ofsaid substrate, and λ is a wavelength of said surface acoustic wave.

(21) The surface acoustic wave device of (1), wherein:

when a cut angle of said substrate cut out of the langasite singlecrystal and a propagation direction of a surface acoustic wave on saidsubstrate are represented in terms of Euler angles (φ, θ, ψ), φ, θ, andψ lie within the following area III-7:

Area III-7

-5°≦φ≦5°

85°≦θ≦95°

25°≦ψ<-50°

(22) The surface acoustic wave device of (21), which satisfies:

    0<h/λ≦0.5

where h is a thickness of said piezoelectric film on said surface ofsaid substrate, and λ is a wavelength of said surface acoustic wave.

(23) The surface acoustic wave device of (1), wherein:

when a cut angle of said substrate cut out of the langasite singlecrystal and a propagation direction of a surface acoustic wave on saidsubstrate are represented in terms of Euler angles (φ, θ, ψ), φ, θ, andψ lie within the following area III-8:

Area III-8

-5°≦φ≦5°

85°≦θ≦95°

35°≦ψ<50°

(24) The surface acoustic wave device of (23), which satisfies:

    0<h/λ≦0.45

where h is a thickness of said piezoelectric film on said surface ofsaid substrate, and λ is a wavelength of said surface acoustic wave.

(25) The surface acoustic wave device of (1), wherein:

when a cut angle of said substrate cut out of the langasite singlecrystal and a propagation direction of a surface acoustic wave on saidsubstrate are represented in terms of Euler angles (φ, θ, ψ), φ, θ, andψ lie within the following area III-9:

Area III-9

-5°≦φ≦5°

85°≦θ≦95°

50°≦ψ<70°

(26) The surface acoustic wave device of (25), which satisfies:

    0<h/λ≦0.05

where h is a thickness of said piezoelectric film on said surface ofsaid substrate, and λ is a wavelength of said surface acoustic wave.

(27) The surface acoustic wave device of (25), which satisfies:

    h/λ=0.2 to 0.8

where h is a thickness of said piezoelectric film on said surface ofsaid substrate, and λ is a wavelength of said surface acoustic wave.

(28) The surface acoustic wave device of (1), wherein:

when a cut angle of said substrate cut out of the langasite singlecrystal and a propagation direction of a surface acoustic wave on saidsubstrate are represented in terms of Euler angles (φ, θ, ψ), φ, θ, andψ lie within the following area III-10:

Area III-10

-5°≦φ≦5°

85°≦θ≦95°

70°≦ψ<90°

(29) The surface acoustic wave device of (28), which satisfies:

    0<h/λ≦0.05

where h is a thickness of said piezoelectric film on said surface ofsaid substrate, and λ is a wavelength of said surface acoustic wave.

(30) The surface acoustic wave device of (28), which satisfies:

    h/λ=0.25 to 0.8

where h is a thickness of said piezoelectric film on said surface ofsaid substrate, and λ is a wavelength of said surface acoustic wave.

Embodiment 4

(1) A surface acoustic wave device comprising a substrate, an oppositeelectrode film on a surface thereof, a piezoelectric film on saidopposite electrode film, and an interdigital electrode on a surface ofsaid piezoelectric film, wherein:

said substrate is a langasite single crystal belonging to a point group32, and said piezoelectric film is made up of zinc oxide.

(2) The surface acoustic wave device of (1), wherein said piezoelectricfilm has a piezoelectric axis oriented substantially perpendicularlywith respect to said surface of said substrate.

(3) The surface acoustic wave device of (1), wherein:

when a cut angle of said substrate cut out of the langasite singlecrystal and a propagation direction of a surface acoustic wave on saidsubstrate are represented in terms of Euler angles (φ, θ, ψ), φ, θ, andψ lie within the following general area IV:

Area IV

-5°≦φ≦5°

85°≦θ≦95°

-90°≦ψ<90°

(4) The surface acoustic wave device of (1), wherein:

when a cut angle of said substrate cut out of the langasite singlecrystal and a propagation direction of a surface acoustic wave on saidsubstrate are represented in terms of Euler angles (φ, θ, ψ), φ, θ, andψ lie within the following area IV-1:

Area IV-1

-5°≦φ≦5°

85°≦θ≦95°

-90°≦ψ<-70°

(5) The surface acoustic wave device of (4), which satisfies:

    h/λ=0.05 to 0.8

where h is a thickness of said piezoelectric film, and λ is a wavelengthof said surface acoustic wave.

(6) The surface acoustic wave device of (1), wherein:

when a cut angle of said substrate cut out of the langasite singlecrystal and a propagation direction of a surface acoustic wave on saidsubstrate are represented in terms of Euler angles (φ, θ, ψ), φ, θ, andψ lie within the following area IV-2:

Area IV-2

-5°≦φ≦5°

85°≦θ≦95°

-70°≦ψ<-50°

(7) The surface acoustic wave device of (6), which satisfies:

    h/λ=0.05 to 0.8

where h is a thickness of said piezoelectric film, and λ is a wavelengthof said surface acoustic wave.

(8) The surface acoustic wave device of (1), wherein:

when a cut angle of said substrate cut out of the langasite singlecrystal and a propagation direction of a surface acoustic wave on saidsubstrate are represented in terms of Euler angles (φ, θ, ψ), φ, θ, andψ lie within the following area IV-3:

Area IV-3

-5°≦φ≦5°

85°≦θ≦95°

-50°≦ψ<-35°

(9) The surface acoustic wave device of (8), which satisfies:

    h/λ=0.05 to 0.45

where h is a thickness of said piezoelectric film, and λ is a wavelengthof said surface acoustic wave.

(10) The surface acoustic wave device of (1), wherein:

when a cut angle of said substrate cut out of the langasite singlecrystal and a propagation direction of a surface acoustic wave on saidsubstrate are represented in terms of Euler angles (φ, θ, ψ), φ, θ, andψ lie within the following area IV-4:

Area IV-4

-5°≦φ≦5°

85°≦θ≦95°

-35°≦ψ<-25°

(11) The surface acoustic wave device of (10), which satisfies:

    h/λ=0.05 to 0.5

where h is a thickness of said piezoelectric film, and λ is a wavelengthof said surface acoustic wave.

(12) The surface acoustic wave device of (1), wherein:

when a cut angle of said substrate cut out of the langasite singlecrystal and a propagation direction of a surface acoustic wave on saidsubstrate are represented in terms of Euler angles (φ, θ, ψ), φ, θ, andψ lie within the following area IV-5:

Area IV-5

-5°≦φ≦5°

85°≦θ≦95°

-25°≦ψ≦-10°(13) The surface acoustic wave device of (12), whichsatisfies:

    h/λ=0.05 to 0.45

where h is a thickness of said piezoelectric film, and λ is a wavelengthof said surface acoustic wave.

(14) The surface acoustic wave device of (1), wherein:

when a cut angle of said substrate cut out of the langasite singlecrystal and a propagation direction of a surface acoustic wave on saidsubstrate are represented in terms of Euler angles (φ, θ, ψ), φ, θ, andψ lie within the following area IV-6:

Area IV-6

-5°≦φ≦5°

85°≦θ≦95°

10°≦ψ<25°

(15) The surface acoustic wave device of (14), which satisfies:

    h/λ=0.05 to 0.45

where h is a thickness of said piezoelectric film on said surface ofsaid substrate, and X is a wavelength of said surface acoustic wave.

(16) The surface acoustic wave device of (1), wherein:

when a cut angle of said substrate cut out of the langasite singlecrystal and a propagation direction of a surface acoustic wave on saidsubstrate are represented in terms of Euler angles (φ, θ, ψ), φ, θ, andψ lie within the following area IV-7:

Area IV-7

-5°≦φ≦5°

85°≦θ≦95°

25°≦ψ<35°

(17) The surface acoustic wave device of (16), which satisfies:

    h/λ=0.05 to 0.5

where h is a thickness of said piezoelectric film, and λ is a wavelengthof said surface acoustic wave.

(18) The surface acoustic wave device of (1), wherein:

when a cut angle of said substrate cut out of the langasite singlecrystal and a propagation direction of a surface acoustic wave on saidsubstrate are represented in terms of Euler angles (φ, θ, ψ), φ, θ, andψ lie within the following area IV-8:

Area IV-8

-5°≦φ≦5°

85°≦θ≦95°

35°≦ψ<50°

(19) The surface acoustic wave device of (18), which satisfies:

    h/λ=0.05 to 0.45

where h is a thickness of said piezoelectric film, and λ is a wavelengthof said surface acoustic wave.

(20) The surface acoustic wave device of (1), wherein:

when a cut angle of said substrate cut out of the langasite singlecrystal and a propagation direction of a surface acoustic wave on saidsubstrate are represented in terms of Euler angles (φ, θ, ψ), φ, θ, andψ lie within the following area IV-9:

Area IV-9

-5°≦φ≦5°

85°≦θ≦95°

50°≦ψ<70°

(21) The surface acoustic wave device of (20), which satisfies:

    h/λ=0.05 to 0.8

where h is a thickness of said piezoelectric film, and λ is a wavelengthof said surface acoustic wave.

(22) The surface acoustic wave device of (1), wherein:

when a cut angle of said substrate cut out of the langasite singlecrystal and a propagation direction of a surface acoustic wave on saidsubstrate are represented in terms of Euler angles (φ, θ, ψ), φ, θ, andψ lie within the following area IV-10:

Area IV-10

-5°≦φ≦5°

85°≦θ≦95°

70°≦ψ<90°

(23) The surface acoustic wave device of (22), which satisfies:

    h/λ=0.05 to 0.8

where h is a thickness of said piezoelectric film, and λ is a wavelengthof said surface acoustic wave.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a sectional representation of one exemplary construction ofthe surface acoustic wave device according to embodiment 1 of theinvention.

FIGS. 2A, 2B, and 2C are generally illustrative of varying normalizedthickness h/λ of ZnO film vs. characteristic change relations for asurface acoustic wave device comprising a langasite single crystalsubstrate making use of area I-1 and a ZnO film formed on the surfacethereof, and are a graph for SAW velocity changes, a graph forelectromechanical coupling factor k² changes, and a graph for TCV (atemperature coefficient of SAW velocity) changes, respectively.

FIGS. 3A, 3B, and 3C are generally illustrative of varying normalizedthickness h/λ of ZnO film vs. characteristic change relations for asurface acoustic wave device comprising a langasite single crystalsubstrate making use of area I-2 and a ZnO film formed on the surfacethereof, and are a graph for SAW velocity changes, a graph forelectromechanical coupling factor k² changes, and a graph for TCV (atemperature coefficient of SAW velocity) changes, respectively.

FIGS. 4A, 4B, and 4C are generally illustrative of varying normalizedthickness h/λ of ZnO film vs. characteristic change relations for asurface acoustic wave device comprising a langasite single crystalsubstrate making use of area I-3 and a ZnO film formed on the surfacethereof, and are a graph for SAW velocity changes, a graph forelectromechanical coupling factor k² changes, and a graph for TCV (atemperature coefficient of SAW velocity) changes, respectively.

FIGS. 5A, 5B, and 5C are generally illustrative of varying normalizedthickness h/λ of ZnO film vs. characteristic change relations for asurface acoustic wave device comprising a langasite single crystalsubstrate making use of area I-4 and a ZnO film formed on the surfacethereof, and are a graph for SAW velocity changes, a graph forelectromechanical coupling factor k² changes, and a graph for TCV (atemperature coefficient of SAW velocity) changes, respectively.

FIGS. 6A, 6B, and 6C are generally illustrative of varying normalizedthickness h/λ of ZnO film vs. characteristic change relations for asurface acoustic wave device comprising a langasite single crystalsubstrate making use of area I-5 and a ZnO film formed on the surfacethereof, and are a graph for SAW velocity changes, a graph forelectromechanical coupling factor k² changes, and a graph for TCV (atemperature coefficient of SAW velocity) changes, respectively.

FIGS. 7A, 7B, and 7C are generally illustrative of varying normalizedthickness h/λ of ZnO film vs. characteristic change relations for asurface acoustic wave device comprising a langasite single crystalsubstrate making use of area I-6 and a ZnO film formed on the surfacethereof, and are a graph for SAW velocity changes, a graph forelectromechanical coupling factor k² changes, and a graph for TCV (atemperature coefficient of SAW velocity) changes, respectively.

FIGS. 8A, 8B, and 8C are generally illustrative of varying normalizedthickness h/λ of ZnO film vs. characteristic change relations for asurface acoustic wave device comprising a langasite single crystalsubstrate making use of area I-7 and a ZnO film formed on the surfacethereof, and are a graph for SAW velocity changes, a graph forelectromechanical coupling factor k² changes, and a graph for TCV (atemperature coefficient of SAW velocity) changes, respectively.

FIGS. 9A, 9B, and 9C are generally illustrative of varying normalizedthickness h/λ of ZnO film vs. characteristic change relations for asurface acoustic wave device comprising a langasite single crystalsubstrate making use of area I-8 and a ZnO film formed on the surfacethereof, and are a graph for SAW velocity changes, a graph forelectromechanical coupling factor k² changes, and a graph for TCV (atemperature coefficient of SAW velocity) changes, respectively.

FIGS. 10A, 10B, and 10C are generally illustrative of varying normalizedthickness h/λ of ZnO film vs. characteristic change relations for asurface acoustic wave device comprising a langasite single crystalsubstrate making use of area I-9 and a ZnO film formed on the surfacethereof, and are a graph for SAW velocity changes, a graph forelectromechanical coupling factor k² changes, and a graph for TCV (atemperature coefficient of SAW velocity) changes, respectively.

FIGS. 11A, 11B, and 11C are generally illustrative of varying normalizedthickness h/λ of ZnO film vs. characteristic change relations for asurface acoustic wave device comprising a langasite single crystalsubstrate making use of area I-10 and a ZnO film formed on the surfacethereof, and are a graph for SAW velocity changes, a graph forelectromechanical coupling factor k² changes, and a graph for TCV (atemperature coefficient of SAW velocity) changes, respectively.

FIG. 12 is a graph illustrative of TCV (a temperature coefficient of SAWvelocity) changes for a surface acoustic wave device comprising alangasite single crystal substrate making use of area I-1 and a ZnO filmformed on the surface thereof, when the normalized thickness h/λ of theZnO film and ψ for defining the propagation direction of a surfaceacoustic wave are varied.

FIG. 13 is a graph illustrative of electromechanical coupling factor k²changes for a surface acoustic wave device comprising a langasite singlecrystal substrate making use of area I-1 and a ZnO film formed on thesurface thereof, when the normalized thickness h/λ of the ZnO film and ψfor defining the propagation direction of a surface acoustic wave arevaried.

FIG. 14 is a graph illustrative of TCV (a temperature coefficient of SAWvelocity) changes for a surface acoustic wave device comprising alangasite single crystal substrate making use of area I-10 and a ZnOfilm formed on the surface thereof, when the normalized thickness h/λ ofthe ZnO film and ψ for defining the propagation direction of a surfaceacoustic wave are varied.

FIG. 15 is a graph illustrative of electromechanical coupling factor k²changes for a surface acoustic wave device comprising a langasite singlecrystal substrate making use of area I-10 and a ZnO film formed on thesurface thereof, when the normalized thickness h/λ of the ZnO film and ψfor defining the propagation direction of a surface acoustic wave arevaried.

FIG. 16 is a sectional representation of one exemplary construction ofthe surface acoustic wave device according to embodiment 2 of theinvention.

FIGS. 17A, 17B, and 17C are generally illustrative of varying normalizedthickness h/λ of ZnO film vs. characteristic change relations for asurface acoustic wave device comprising a langasite single crystalsubstrate making use of area II-1, and a ZnO film and an interdigitalelectrode formed on the surface thereof, and are a graph for SAWvelocity changes, a graph for electromechanical coupling factor k2changes, and a graph for TCV (a temperature coefficient of SAW velocity)changes, respectively.

FIGS. 18A, 18B, and 18C are generally illustrative of varying normalizedthickness h/λ of ZnO film vs. characteristic change relations for asurface acoustic wave device comprising a langasite single crystalsubstrate making use of area II-2, and a ZnO film and an interdigitalelectrode formed on the surface thereof, and are a graph for SAWvelocity changes, a graph for electromechanical coupling factor k²changes, and a graph for TCV (a temperature coefficient of SAW velocity)changes, respectively.

FIGS. 19A, 19B, and 19C are generally illustrative of varying normalizedthickness h/λ of ZnO film vs. characteristic change relations for asurface acoustic wave device comprising a langasite single crystalsubstrate making use of area II-3, and a ZnO film and an interdigitalelectrode formed on the surface thereof, and are a graph for SAWvelocity changes, a graph for electromechanical coupling factor k²changes, and a graph for TCV (a temperature coefficient of SAW velocity)changes, respectively.

FIGS. 20A, 20B, and 20C are generally illustrative of varying normalizedthickness h/λ of ZnO film vs. characteristic change relations for asurface acoustic wave device comprising a langasite single crystalsubstrate making use of area II-4, and a ZnO film and an interdigitalelectrode formed on the surface thereof, and are a graph for SAWvelocity changes, a graph for electromechanical coupling factor k2changes, and a graph for TCV (a temperature coefficient of SAW velocity)changes, respectively.

FIGS. 21A, 21B, and 21C are generally illustrative of varying normalizedthickness h/λ of ZnO film vs. characteristic change relations for asurface acoustic wave device comprising a langasite single crystalsubstrate making use of area II-5, and a ZnO film and an interdigitalelectrode formed on the surface thereof, and are a graph for SAWvelocity changes, a graph for electromechanical coupling factor k²changes, and a graph for TCV (a temperature coefficient of SAW velocity)changes, respectively.

FIGS. 22A, 22B, and 22C are generally illustrative of varying normalizedthickness h/λ of ZnO film vs. characteristic change relations for asurface acoustic wave device comprising a langasite single crystalsubstrate making use of area II-6, and a ZnO film and an interdigitalelectrode formed on the surface thereof, and are a graph for SAWvelocity changes, a graph for electromechanical coupling factor k²changes, and a graph for TCV (a temperature coefficient of SAW velocity)changes, respectively.

FIGS. 23A, 23B, and 23C are generally illustrative of varying normalizedthickness h/λ of ZnO film vs. characteristic change relations for asurface acoustic wave device comprising a langasite single crystalsubstrate making use of area II-7, and a ZnO film and an interdigitalelectrode formed on the surface thereof, and are a graph for SAWvelocity changes, a graph for electromechanical coupling factor k²changes, and a graph for TCV (a temperature coefficient of SAW velocity)changes, respectively.

FIGS. 24A, 24B, and 24C are generally illustrative of varying normalizedthickness h/λ of ZnO film vs. characteristic change relations for asurface acoustic wave device comprising a langasite single crystalsubstrate making use of area II-8, and a ZnO film and an interdigitalelectrode formed on the surface thereof, and are a graph for SAWvelocity changes, a graph for electromechanical coupling factor k²changes, and a graph for TCV (a temperature coefficient of SAW velocity)changes, respectively.

FIGS. 25A, 25B, and 25C are generally illustrative of varying normalizedthickness h/λ of ZnO film vs. characteristic change relations for asurface acoustic wave device comprising a langasite single crystalsubstrate making use of area II-9, and a ZnO film and an interdigitalelectrode formed on the surface thereof, and are a graph for SAWvelocity changes, a graph for electromechanical coupling factor k²changes, and a graph for TCV (a temperature coefficient of SAW velocity)changes, respectively.

FIGS. 26A, 26B, and 26C are generally illustrative of varying normalizedthickness h/λ of ZnO film vs. characteristic change relations for asurface acoustic wave device comprising a langasite single crystalsubstrate making use of area II-10, and a ZnO film and an interdigitalelectrode formed on the surface thereof, and are a graph for SAWvelocity changes, a graph for electromechanical coupling factor k²changes, and a graph for TCV (a temperature coefficient of SAW velocity)changes, respectively.

FIG. 27 is a graph illustrative of TCV (a temperature coefficient of SAWvelocity) changes for a surface acoustic wave device comprising alangasite single crystal substrate making use of area II-1, and a ZnOfilm and an interdigital electrode formed on the surface thereof, whenthe normalized thickness h/λ of the ZnO film and ψ for defining thepropagation direction of a surface acoustic wave are varied.

FIG. 28 is a graph illustrative of electromechanical coupling factor k²changes for a surface acoustic wave device comprising a langasite singlecrystal substrate making use of area II-1, and a ZnO film and aninterdigital electrode formed on the surface thereof, when thenormalized thickness h/λ of the ZnO film and ψ for defining thepropagation direction of a surface acoustic wave are varied.

FIG. 29 is a graph illustrative of TCV (a temperature coefficient of SAWvelocity) changes for a surface acoustic wave device comprising alangasite single crystal substrate making use of area II-10, and a ZnOfilm and an interdigital electrode formed on the surface thereof, whenthe normalized thickness h/λ of the ZnO film and ψ for defining thepropagation direction of a surface acoustic wave are varied.

FIG. 30 is a graph illustrative of electromechanical coupling factor k²changes for a surface acoustic wave device comprising a langasite singlecrystal substrate making use of area II-10, and a ZnO film and aninterdigital electrode formed on the surface thereof, when thenormalized thickness h/λ of the ZnO film and ψ for defining thepropagation direction of a surface acoustic wave are varied.

FIG. 31 is a sectional representation of one exemplary construction ofthe surface acoustic wave device according to embodiment 3 of theinvention.

FIGS. 32A, 32B, and 32C are generally illustrative of varying normalizedthickness h/λ of ZnO film vs. characteristic change relations for asurface acoustic wave device comprising a langasite single crystalsubstrate making use of area III-1, and an interdigital electrode, a ZnOfilm and an opposite electrode film formed on the surface thereof inthis order, and are a graph for SAW velocity changes, a graph forelectromechanical coupling factor k² changes, and a graph for TCV (atemperature coefficient of SAW velocity) changes, respectively.

FIGS. 33A, 33B, and 33C are generally illustrative of varying normalizedthickness h/λ of ZnO film vs. characteristic change relations for asurface acoustic wave device comprising a langasite single crystalsubstrate making use of area III-2, and an interdigital electrode, a ZnOfilm and an opposite electrode film formed on the surface thereof inthis order, and are a graph for SAW velocity changes, a graph forelectromechanical coupling factor k² changes, and a graph for TCV (atemperature coefficient of SAW velocity) changes, respectively.

FIGS. 34A, 34B, and 34C are generally illustrative of varying normalizedthickness h/λ of ZnO film vs. characteristic change relations for asurface acoustic wave device comprising a langasite single crystalsubstrate making use of area III-3, and an interdigital electrode, a ZnOfilm and an opposite electrode film formed on the surface thereof inthis order, and are a graph for SAW velocity changes, a graph forelectromechanical coupling factor k² changes, and a graph for TCV (atemperature coefficient of SAW velocity) changes, respectively.

FIGS. 35A, 35B, and 35C are generally illustrative of varying normalizedthickness h/λ of ZnO film vs. characteristic change relations for asurface acoustic wave device comprising a langasite single crystalsubstrate making use of area III-4, and an interdigital electrode, a ZnOfilm and an opposite electrode film formed on the surface thereof inthis order, and are a graph for SAW velocity changes, a graph forelectromechanical coupling factor k² changes, and a graph for TCV (atemperature coefficient of SAW velocity) changes, respectively.

FIGS. 36A, 36B, and 36C are generally illustrative of varying normalizedthickness h/λ of ZnO film vs. characteristic change relations for asurface acoustic wave device comprising a langasite single crystalsubstrate making use of area III-5, and an interdigital electrode, a ZnOfilm and an opposite electrode film formed on the surface thereof inthis order, and are a graph for SAW velocity changes, a graph forelectromechanical coupling factor k² changes, and a graph for TCV (atemperature coefficient of SAW velocity) changes, respectively.

FIGS. 37A, 37B, and 37C are generally illustrative of varying normalizedthickness h/λ of ZnO film vs. characteristic change relations for asurface acoustic wave device comprising a langasite single crystalsubstrate making use of area III-6, and an interdigital electrode, a ZnOfilm and an opposite electrode film formed on the surface thereof, andare a graph for SAW velocity changes, a graph for electromechanicalcoupling factor k² changes, and a graph for TCV (a temperaturecoefficient of SAW velocity) changes, respectively.

FIGS. 38A, 38B, and 38C are generally illustrative of varying normalizedthickness h/λ of ZnO film vs. characteristic change relations for asurface acoustic wave device comprising a langasite single crystalsubstrate making use of area III-7, and an interdigital electrode, a ZnOfilm and an opposite electrode film formed on the surface thereof inthis order, and are a graph for SAW velocity changes, a graph forelectromechanical coupling factor k² changes, and a graph for TCV (atemperature coefficient of SAW velocity) changes, respectively.

FIGS. 39A, 39B, and 39C are generally illustrative of varying normalizedthickness h/λ of ZnO film vs. characteristic change relations for asurface acoustic wave device comprising a langasite single crystalsubstrate making use of area III-8, and an interdigital electrode, a ZnOfilm and an opposite electrode film formed on the surface thereof inthis order, and are a graph for SAW velocity changes, a graph forelectromechanical coupling factor k² changes, and a graph for TCV (atemperature coefficient of SAW velocity) changes, respectively.

FIGS. 40A, 40B, and 40C are generally illustrative of varying normalizedthickness h/λ of ZnO film vs. characteristic change relations for asurface acoustic wave device comprising a langasite single crystalsubstrate making use of area III-9, and an interdigital electrode, a ZnOfilm and an opposite electrode film formed on the surface thereof inthis order, and are a graph for SAW velocity changes, a graph forelectromechanical coupling factor k² changes, and a graph for TCV (atemperature coefficient of SAW velocity) changes, respectively.

FIGS. 41A, 41B, and 41C are generally illustrative of varying normalizedthickness h/λ of ZnO film vs. characteristic change relations for asurface acoustic wave device comprising a langasite single crystalsubstrate making use of area III-10, and an interdigital electrode, aZnO film and an opposite electrode film formed on the surface thereof inthis order, and are a graph for SAW velocity changes, a graph forelectromechanical coupling factor k² changes, and a graph for TCV (atemperature coefficient of SAW velocity) changes, respectively.

FIG. 42 is a graph illustrative of TCV (a temperature coefficient of SAWvelocity) changes for a surface acoustic wave device comprising alangasite single crystal substrate making use of area III-1, and aninterdigital electrode, a ZnO and an opposite electrode film formed onthe surface thereof in this order, when the normalized thickness h/λ ofthe ZnO film and ψ for defining the propagation direction of a surfaceacoustic wave are varied.

FIG. 43 is a graph illustrative of electromechanical coupling factor k²changes for a surface acoustic wave device comprising a langasite singlecrystal substrate making use of area III-1, and an interdigitalelectrode, a ZnO film and an opposite electrode film formed on thesurface thereof, when the normalized thickness h/λ of the ZnO film and ψfor defining the propagation direction of a surface acoustic wave arevaried.

FIG. 44 is a graph illustrative of TCV (a temperature coefficient of SAWvelocity) changes for a surface acoustic wave device comprising alangasite single crystal substrate making use of area III-10, and aninterdigital electrode, a ZnO and an opposite electrode film formed onthe surface thereof in this order, when the normalized thickness h/λ ofthe ZnO film and ψ for defining the propagation direction of a surfaceacoustic wave are varied.

FIG. 45 is a graph illustrative of electromechanical coupling factor k²changes for a surface acoustic wave device comprising a langasite singlecrystal substrate making use of area III-10, and an interdigialelectrode, a ZnO film and an opposite electrode film formed on thesurface thereof in this order, when the normalized thickness h/λ of theZnO film and ψ for defining the propagation direction of a surfaceacoustic wave are varied.

FIG. 46 is a sectional representation of one exemplary construction ofthe surface acoustic wave device according to embodiment 4 of theinvention.

FIGS. 47A, 47B, and 47C are generally illustrative of varying normalizedthickness h/λ of ZnO film vs. characteristic change relations for asurface acoustic wave device comprising a langasite single crystalsubstrate making use of area IV-1, and an opposite electrode film, a ZnOfilm and an interdigital electrode formed on the surface thereof in thisorder, and are a graph for SAW velocity changes, a graph forelectromechanical coupling factor k² changes, and a graph for TCV (atemperature coefficient of SAW velocity) changes, respectively.

FIGS. 48A, 48B, and 48C are generally illustrative of varying normalizedthickness h/λ of ZnO film vs. characteristic change relations for asurface acoustic wave device comprising a langasite single crystalsubstrate making use of area IV-2, and an opposite electrode film, a ZnOfilm and an interdigital electrode formed on the surface thereof in thisorder, and are a graph for SAW velocity changes, a graph forelectromechanical coupling factor k² changes, and a graph for TCV (atemperature coefficient of SAW velocity) changes, respectively.

FIGS. 49A, 49B, and 49C are generally illustrative of varying normalizedthickness h/λ of ZnO film vs. characteristic change relations for asurface acoustic wave device comprising a langasite single crystalsubstrate making use of area IV-3, and an opposite electrode film, a ZnOfilm and an interdigital electrode formed on the surface thereof in thisorder, and are a graph for SAW velocity changes, a graph forelectromechanical coupling factor k2 changes, and a graph for TCV (atemperature coefficient of SAW velocity) changes, respectively.

FIGS. 50A, 50B, and 50C are generally illustrative of varying normalizedthickness h/λ of ZnO film vs. characteristic change relations for asurface acoustic wave device comprising a langasite single crystalsubstrate making use of area IV-4, and an opposite electrode film, a ZnOfilm and an interdigital electrode formed on the surface thereof in thisorder, and are a graph for SAW velocity changes, a graph forelectromechanical coupling factor k² changes, and a graph for TCV (atemperature coefficient of SAW velocity) changes, respectively.

FIGS. 51A, 51B, and 51C are generally illustrative of varying normalizedthickness h/λ of ZnO film vs. characteristic change relations for asurface acoustic wave device comprising a langasite single crystalsubstrate making use of area IV-5, and an opposite electrode film, a ZnOfilm and an interdigital electrode formed on the surface thereof in thisorder, and are a graph for SAW velocity changes, a graph forelectromechanical coupling factor k² changes, and a graph for TCV (atemperature coefficient of SAW velocity) changes, respectively.

FIGS. 52A, 52B, and 52C are generally illustrative of varying normalizedthickness h/λ of ZnO film vs. characteristic change relations for asurface acoustic wave device comprising a langasite single crystalsubstrate making use of area IV-6, and an opposite electrode film, a ZnOfilm and an interdigital electrode formed on the surface thereof in thisorder, and are a graph for SAW velocity changes, a graph forelectromechanical coupling factor k² changes, and a graph for TCV (atemperature coefficient of SAW velocity) changes, respectively.

FIGS. 53A, 53B, and 53C are generally illustrative of varying normalizedthickness h/λ of ZnO film vs. characteristic change relations for asurface acoustic wave device comprising a langasite single crystalsubstrate making use of area IV-7, and an opposite electrode film, a ZnOfilm and an interdigital electrode formed on the surface thereof in thisorder, and are a graph for SAW velocity changes, a graph forelectromechanical coupling factor k² changes, and a graph for TCV (atemperature coefficient of SAW velocity) changes, respectively.

FIGS. 54A, 54B, and 54C are generally illustrative of varying normalizedthickness h/λ of ZnO film vs. characteristic change relations for asurface acoustic wave device comprising a langasite single crystalsubstrate making use of area IV-8, and an opposite electrode film, a ZnOfilm and an interdigital electrode formed on the surface thereof in thisorder, and are a graph for SAW velocity changes, a graph forelectromechanical coupling factor k² changes, and a graph for TCV (atemperature coefficient of SAW velocity) changes, respectively.

FIGS. 55A, 55B, and 55C are generally illustrative of varying normalizedthickness h/λ of ZnO film vs. characteristic change relations for asurface acoustic wave device comprising a langasite single crystalsubstrate making use of area IV-9, and an opposite electrode film, a ZnOfilm and an interdigital electrode formed on the surface thereof in thisorder, and are a graph for SAW velocity changes, a graph forelectromechanical coupling factor k² changes, and a graph for TCV (atemperature coefficient of SAW velocity) changes, respectively.

FIGS. 56A, 56B, and 56C are generally illustrative of varying normalizedthickness h/λ of ZnO film vs. characteristic change relations for asurface acoustic wave device comprising a langasite single crystalsubstrate making use of area IV-10, and an opposite electrode film, aZnO film and an interdigital electrode formed on the surface thereof inthis order, and are a graph for SAW velocity changes, a graph forelectromechanical coupling factor k2 changes, and a graph for TCV (atemperature coefficient of SAW velocity) changes, respectively.

FIG. 57 is a graph illustrative of TCV (a temperature coefficient of SAWvelocity) changes for a surface acoustic wave device comprising alangasite single crystal substrate making use of area IV-1, and anopposite electrode film, a ZnO film and an interdigital electrode formedon the surface thereof in this order, when the normalized thickness h/λof the ZnO film and ψ for defining the propagation direction of asurface acoustic wave are varied.

FIG. 58 is a graph illustrative of electromechanical coupling factor k²changes for a surface acoustic wave device comprising a langasite singlecrystal substrate making use of area IV-1, and an opposite electrodefilm, a ZnO film and an interdigital electrode formed on the surfacethereof in this order, when the normalized thickness h/λ of the ZnO filmand ψ for defining the propagation direction of a surface acoustic waveare varied.

FIG. 59 is a graph illustrative of TCV (a temperature coefficient of SAWvelocity) changes for a surface acoustic wave device comprising alangasite single crystal substrate making use of area IV-10, and anopposite electrode film, a ZnO film and an interdigital electrode formedon the surface thereof in this order, when the normalized thickness h/λof the ZnO film and ψ for defining the propagation direction of asurface acoustic wave are varied.

FIG. 60 is a graph illustrative of electromechanical coupling factor k2changes for a surface acoustic wave device comprising a langasite singlecrystal substrate making use of area IV-10, and an opposite electrodefilm, a ZnO film and an interdigital electrode formed on the surfacethereof in this order, when the normalized thickness h/λ of the ZnO filmand ψ for defining the propagation direction of a surface acoustic waveare varied.

DESCRIPTION OF THE PREFERRED EMBODIMENTS

Embodiment 1

In embodiment 1, the langasite single crystal substrate is used as thesubstrate material for a surface acoustic wave device, and φ, θ, and ψrepresenting the cut angle of the substrate cut out of the langasitesingle crystal and the propagation direction of surface acoustic waveson the substrate are selected from within the general area I. This makesit possible to decrease the SAW velocity, increase the electromechanicalcoupling factor, and decrease the temperature coefficient of SAWvelocity or TCV. In embodiment 1, the piezoelectric film made up of zincoxide is further provided on the surface of the langasite single crystalsubstrate. Then, the thickness of the piezoelectric film is controlleddepending on the cut angle of the substrate out of the langasite singlecrystal and the propagation direction of surface acoustic waves on thesubstrate, thereby achieving a further increase in the electromechanicalcoupling factor and/or a further TCV decrease. This in turn enables toreduce the size of a surface acoustic wave device, and improve thepassband width, and temperature stability of a surface acoustic wavedevice when it is used as a filter. In particular, it is possible toachieve a surface acoustic wave filter best suited for use for mobilecommunication terminal equipment operated at intermediate frequencies.

The areas I-1 to I-10 are encompassed in the general area I, and apreferable thickness range for the piezoelectric film exists per eacharea. At areas I-1 and I-10 of these areas, the absolute value of TCVcan be extremely reduced by a selection of the thickness of thepiezoelectric film and, in some cases, can be substantially reduced tozero. It is thus possible to achieve a surface acoustic wave device thatis particularly excellent in selectivity.

It is known that the electromechanical coupling factor of a surfaceacoustic wave device can be increased by forming a piezoelectric film ofzinc oxide, tantalum oxide, CdS or the like on the surface of apiezoelectric substrate of LiNbO₃ or the like, as typically set forth inJP-A 8-204499. Never until now, however, is a surface acoustic wavedevice comprising a piezoelectric film formed on the surface of alangasite single crystal substrate proposed in the art. Here, thelangasite substrate can have a positive TCV by a selection of the cutangle thereof out of the langasite single crystal and the propagationdirection of surface acoustic waves thereon. On the other hand, the zincoxide film has a negative TCV. When the zinc oxide film is formed on thelangasite substrate, both TCVs are offset each other so that thecombined TCV can be extremely reduced. In this regard, a conventionalpiezoelectric substrate such as an LiNbO₃ substrate having a negativeTCV is not preferable because, when combined with a zinc oxide film, thecombined TCV is greatly shifted to a negative side. According toembodiment 1 wherein a specific langasite substrate is used incombination with the zinc oxide film, it is possible to realize TCVdecreases which would be unachievable by a conventional piezoelectricsubstrate and film combination.

Embodiment 2

In embodiment 2, a piezoelectric film and an interdigital electrode areformed on a piezoelectric substrate in the described order to constructa surface acoustic wave device. A langasite single crystal substrate isused for the piezo- electric substrate while a zinc oxide film is usedfor the piezoelectric film. By the provision of the piezoelectric filmit is possible to increase the electromechanical coupling factor.

It is known that the electromechanical coupling factor can be increasedby forming a piezoelectric film on the surface of a piezoelectricsubstrate, as typically disclosed in JP-A 8-204499. When a conventionalpiezoelectric substrate such as an LiNbO₃ substrate having a negativeTCV is used in combination with a zinc oxide film, however, the combinedTCV is greatly shifted to a negative side.

In the present invention, the langasite single crystal is used for thesubstrate material. The langasite substrate can have a positive TCV by aselection of the cut angle thereof out of the langasite single crystaland the propagation direction of surface acoustic waves thereon. On theother hand, the zinc oxide film has a negative TCV. When the zinc oxidefilm is formed on the langasite substrate, both TCVs are offset eachother so that the combined TCV can be extremely reduced. It is thuspossible to realize a surface acoustic wave device unachievable by aconventional piezoelectric substrate and film combination, i.e., asurface acoustic wave device having an increased electromechanicalcoupling factor and a reduced TCV absolute value.

JP-A 8-204499 also discloses that a piezoelectric film is formed on aninterdigital electrode provided on a piezoelectric substrate. Inembodiment 2 of the invention, on the other hand, the interdigitalelectrode is formed on the piezoelectric film provided on thepiezoelectric substrate. In other words, a homogeneous piezoelectricfilm is obtainable in embodiment 2 because the piezoelectric film growson the flat surface of the langasite single crystal substrate. It isthus possible to eliminate or substantially reduce frequency variationsascribable to the irregularity of the piezoelectric film.

The areas II-1 to II-10 are encompassed in the general area II, and apreferable thickness range for the piezoelectric film exists per eacharea. At areas II-1 and II-10 of these areas, the absolute value of TCVcan be extremely reduced by a selection of the thickness of thepiezoelectric film and, in some cases, can be substantially reduced tozero. It is thus possible to achieve a surface acoustic wave device thatis particularly excellent in selectivity.

Embodiment 3

In embodiment 3 of the invention, a piezoelectric film 4 is provided ona piezoelectric substrate 2 to construct a surface acoustic wave device,as shown in FIG. 31. A langasite single crystal substrate is used forthe piezoelectric substrate while a zinc oxide film is used for thepiezoelectric film, and an opposite electrode film 5 is further providedon the piezoelectric film. By the provision of the piezoelectric filmand opposite electrode film it is possible to increase theelectromechanical coupling factor of the device.

It is known that the electromechanical coupling factor can be increasedby forming a piezoelectric film on the surface of a piezoelectricsubstrate, as typically disclosed in JP-A 8-204499. When a conventionalpiezoelectric substrate such as an LiNbO₃ substrate having a negativeTCV is used in combination with a zinc oxide film, however, the combinedTCV is greatly shifted to a negative side.

It is also known that even when a piezoelectric film is relatively thin,an increased electromechanical coupling factor can be obtained byopposing an interdigital electrode to an opposite electrode film withthe piezoelectric film interleaved between them, as typically describedin "Surface Wave Device, and Its Application", pp. 98-109, published byNikkan Kogyo Shinbun-Sha (1978). In a conventional surface acoustic wavedevice provided with an opposite electrode film, however, anon-piezoelectric substrate such as a glass, silicon or sapphiresubstrate, rather than a piezoelectric substrate, is used. A possibleexplanation of this could be that too large a shift of TCV to a negativeside occurs in the case of a conventional piezoelectric substrate andfilm combination, as mentioned above.

In embodiment 3 of the invention, the langasite single crystal is usedfor the substrate material. The langasite substrate can have a positiveTCV by a selection of the cut angle thereof out of the langasite singlecrystal and the propagation direction of surface acoustic waves thereon.On the other hand, the zinc oxide film has a negative TCV. When the zincoxide film is formed on the langasite substrate, both TCVs are offseteach other so that the combined TCV can be extremely reduced. It is thuspossible to realize a surface acoustic wave device unachievable by aconventional piezoelectric substrate and film combination, i.e., asurface acoustic wave device having an increased electromechanicalcoupling factor and a reduced TCV absolute value.

In embodiment 3 of the invention, φ, θ, and ψ representing the cut angleof the substrate cut out of the langasite single crystal and thepropagation direction of surface acoustic waves on the substrate areselected from within the general area III. This makes it possible todecrease the SAW velocity, increase the electromechanical couplingfactor, and decrease the temperature coefficient of SAW velocity or TCV.Then, the thickness of the piezoelectric film is controlled depending onthe cut angle of the substrate out of the langasite single crystal andthe propagation direction of surface acoustic waves on the substrate,thereby achieving a further increase in the electromechanical couplingfactor and/or a further TCV decrease. This in turn enables to reduce thesize of a surface acoustic wave device, and improve the passband width,and temperature stability of a surface acoustic wave device when it isused as a filter. In particular, it is possible to achieve a surfaceacoustic wave filter best suited for use for mobile communicationterminal equipment operated at intermediate frequencies.

The areas III-1 to III-10 are encompassed in the general area III, and apreferable thickness range for the piezoelectric film exists per eacharea. At areas III-1 and III-10 of these areas, the absolute value ofTCV can be extremely reduced by a selection of the thickness of thepiezoelectric film and, in some cases, can be reduced to substantiallyzero. It is thus possible to achieve a surface acoustic wave device thatis particularly excellent in selectivity.

Embodiment 4

In embodiment 4 of the invention, a piezoelectric film 4 is provided ona piezoelectric substrate 2 to make a surface acoustic wave device, asshown in FIG. 46. A langasite single crystal substrate is used for thepiezoelectric substrate while a zinc oxide film is used for thepiezoelectric film, and an opposite electrode film 5 is furtherinterleaved between the piezoelectric film and the substrate. By theprovision of the piezoelectric film and opposite electrode film it ispossible to increase the electromechanical coupling factor of thedevice.

It is known that the electromechanical coupling factor can be increasedby forming a piezoelectric film on the surface of a piezoelectricsubstrate, as typically disclosed in JP-A 8-204499. When a conventionalpiezoelectric substrate such as an LiNbO₃ substrate having a negativeTCV is used in combination with a zinc oxide film, however, the combinedTCV is greatly shifted to a negative side.

It is also known that even when a piezoelectric film is relatively thin,an increased electromechanical coupling factor can be obtained byopposing an interdigital electrode to an opposite electrode film withthe piezoelectric film interleaved between them, as typically describedin "Surface Wave Device, and Its Application", pp. 98-109, published byNikkan Kogyo Shinbun-Sha (1978). In a conventional surface acoustic wavedevice provided with an opposite electrode film, however, anon-piezoelectric substrate such as a glass, silicon or sapphiresubstrate, rather than a piezoelectric substrate, is used. A possibleexplanation of this could be that too large a shift of TCV to a negativeside occurs in the case of a conventional piezoelectric substrate andfilm combination.

In embodiment 4 of the invention, the langasite single crystal is usedfor a substrate material. The langasite substrate can have a positiveTCV by a selection of the cut angle thereof out of the langasite singlecrystal and the propagation direction of surface acoustic waves thereon.On the other hand, the zinc oxide film has a negative TCV. When the zincoxide film is formed on the langasite substrate, both TCVs are offseteach other so that the combined TCV can be extremely reduced. It is thuspossible to realize a surface acoustic wave device unachievable by aconventional piezoelectric substrate and film combination, i.e., asurface acoustic wave device having an increased electromechanicalcoupling factor and a reduced TCV absolute value.

JP-A 8-204499 also discloses that a piezoelectric film is formed on aninterdigital electrode provided on a piezoelectric substrate. Inembodiment 4 of the invention, on the other hand, the opposite electrodefilm and piezoelectric films are formed on the piezoelectric substrate,and the interdigital electrode is formed on the piezoelectric film. Inother words, a homogeneous piezoelectric film can be obtained inembodiment 4 because the piezoelectric film grows on the flat surface ofthe opposite electrode film. It is thus possible to eliminate orsubstantially reduce frequency variations ascribable to the irregularityof the piezoelectric film.

In embodiment 4 of the invention, φ, θ, and ψ representing the cut angleof the substrate cut out of the langasite single crystal and thepropagation direction of surface acoustic waves on the substrate areselected from within the general area IV. This makes it possible todecrease the SAW velocity, increase the electromechanical couplingfactor, and decrease the temperature coefficient of SAW velocity or TCV.Then, the thickness of the piezoelectric film is controlled depending onthe cut angle of the substrate out of the langasite single crystal andthe propagation direction of surface acoustic waves on the substrate,thereby achieving a further increase in the electromechanical couplingfactor and/or a further TCV decrease. This in turn enables to reduce thesize of a surface acoustic wave device, and improve the passband width,and temperature stability of a surface acoustic wave device when it isused as a filter. In particular, it is possible to achieve a surfaceacoustic wave filter best suited for use for mobile communicationterminal equipment operated at intermediate frequencies.

The areas IV-1 to IV-10 are encompassed in the general area IV, and apreferable thickness range for the piezoelectric film exists per eacharea. At areas IV-1 and IV-10 of these areas, the absolute value of TCVcan be extremely reduced by a selection of the thickness of thepiezoelectric film and, in some cases, can be substantially reduced tozero. It is thus possible to achieve a surface acoustic wave device thatis particularly excellent in selectivity.

It is here to be noted that "NUMERICAL AND EXPERIMENTAL INVESTIGATIONSAW IN LANGASITE", 1995 IEEE ULTRASONICS SYMPOSIUM, Vol. 1,389(reference publication 1), for instance, gives a report of numericallycalculated SAW velocity, k² /2, TCD (the temperature coefficient of SAWdelay time), etc. for langasite single crystal substrates found to be

(0°, 30°, 90°)

(0°, 53°, 90°)

(0°, 61°, 0°)

(0°, 147°, 22°)

(0°, 147°, 18°)

(0°, 32°, 40°)

(0°, 156°, 0°)

(0°, θ°, 0°)

(0°, 25°, ψ)

when the cut angles of the substrates cut out of the langasite singlecrystal and the propagation direction of surface acoustic waves on thesubstrates are represented in terms of Euler angles (φ, θ, ψ). Also,"Effects of Electric Field and of Mechanical Pressure on SurfaceAcoustic Waves Propagation in La₃ Ga₅ SiO₁₄ Piezoelectric SingleCrystals", 1995 IEEE ULTRASONICS SYMPOSIUM, Vol. 1, 409 (referencepublication 2) gives a report of numerically calculated k² or the likefor substrates represented in terms of Euler angles (φ, θ, ψ), that is,

(90°, 90°, ψ)

(0°, 90°, ψ)

(0°, 0°, ψ)

(0°, θ, 0°)

(90°, θ, 0°)

(φ, 90°, 0°)

Further, an article "A study on SAW propagation characteristics on alangasite crystal plate" in "The 17th Symposium Preprint on theFundamentals and Applications of Ultrasonic Electronics" (referencepublication 3) presents a report of numerically calculated k², TCD, etc.for substrates represented in terms of Euler angles (φ, θ, ψ), viz.,

(90°, 90°, ψ)

and of actually found TCD for substrates represented in terms of

(0°, 0°, 90°)

(90°, 90°, 175°)

(90°, 90°, 25°)

Fruthermore, an article "Propagation direction dependence of Rayleighwaves on a langasite plate" on page 21 of materials distributed at the51th Study Meeting in the 150th Committee of Surface Acoustic WaveDevice Technology, the Japan Society for the Promotion of Science(reference publication 4) presents a report of numerically calculatedk², etc. for substrates represented in terms of Euler angles (φ, θ, ψ),viz.,

(0°, 0°, ψ)

(90°, 90°, ψ)

and of TCD calculated from series resonance frequencies actually foundfor substrates represented in terms of

(0°, 0°, 90°)

(90°, 90°, 175°)

(90°, 90°, 15°)

(90°, 90°, 21°)

(90°, 90°, 25°)

This reference publication 4 was issued on Jan. 27, 1997, or after thefiling of a basic application of this application. Still further, anarticle "Propagation characteristics of surface acoustic waves on La₃Ga₅ SiO₁₄ " in "The 17th Symposium Preprint on the Fundamentals andApplications of Ultrasonic Electronics" (reference publication 5)presents a report of numerically calculated k², etc. for substratesrepresented in terms of Euler angles (φ, θ, ψ), viz.,

(90°, 90°, ψ)

(0°, 90°, ψ)

(0°, 0°, ψ)

(0°, θ, 0°)

Each of the aforesaid reference publications refers to the properties ofthe langasite single crystal substrate per se. Nowhere in thesepublications, however, is the provision of a piezoelectric film formedof zinc oxide on this langasite single crystal substrate disclosedwhatsoever. In the present invention, the piezoelectric film formed ofzinc oxide is controlled to the optimum thickness depending on the cutangle of the substrate out of the langasite single crystal and thepropagation direction of surface acoustic waves on the substrate for thepurpose of achieving a further increase in the electromechanicalcoupling factor and/or a further TCV decrease. Thus, the presentinvention cannot be easily anticipated by the reference publicationsmentioned above.

Embodiment 1

One exemplary architecture of the surface acoustic wave device accordingto embodiment 1 is shown in FIG. 1. This surface acoustic wave devicecomprises a substrate 2, a set of an input side interdigital electrode 3and an output side interdigital electrode 3 formed on the surface of thesubstrate 2, and a piezoelectric film 4 provided to cover the substrate2 and the interdigital electrodes 3 and 3. In each embodiment of theinvention, a langasite single crystal is used for the substrate 2. Thelangasite single crystal is a crystal type belonging to a point group32. In each embodiment of the invention, zinc oxide (ZnO) is used forthe piezoelectric film 4. The piezoelectric film has a piezoelectricaxis oriented substantially perpendicularly with respect to the surfaceof the substrate.

In FIG. 1, x, y, and z-axes are perpendicular to one another. The x, andy-axes lie in a plane direction of the substrate 2, and the x-axisdefines a propagation direction of surface acoustic waves. The z-axisperpendicular to the substrate plane defines a cut angle (cut plane) ofthe substrate cut out of the single crystal. Relations between these x,y and z-axes and the X, Y and Z-axes of the langasite single crystal maybe represented in terms of Euler angles (φ, θ, ψ).

When, in the surface acoustic wave device according to embodiment 1, thecut angle of the substrate out of the langasite single crystal and thepropagation direction of surface acoustic waves are represented in termsof Euler angles (φ, θ, ψ), φ, θ, and ψ exist in each of the areasmentioned above.

By selecting φ, θ and ψ from the general area I and providing apiezoelectric film of suitable thickness, it is possible to decrease theSAW velocity, increase the electromechanical coupling factor, anddecrease the temperature coefficient of SAW velocity or TCV. This inturn enables to reduce the size of a surface acoustic wave device, andimprove the passband width, and temperature stability of a surfaceacoustic wave device when it is used as a filter. In particular, it ispossible to achieve a surface acoustic wave filter best suited for usefor mobile communication terminal equipment operated at intermediatefrequencies. More illustratively, the temperature coefficient of SAWvelocity or TCV of the substrate can be in the range of -35 to 60 ppm/°C., the SAW velocity of the substrate can be up to 2,900 m/s, and thecoupling factor of the substrate can be 0.1% or higher. In some cases,much better characteristics may be obtained.

At areas I-1, I-6, I-7, I-8, I-9, and I-10, a broader passband surfaceacoustic wave device can be achieved because a coupling factor of 0.4%or higher can be attained. At areas I-1, I-7, and I-10, a much broadersurface acoutstic wave device can be achieved because a coupling factorof 0.7% or higher can be attained.

At areas I-1, and I-10, TCV can be extremely reduced and, in some cases,can be reduced to zero, and so a surface acoustic wave device havinggood-enough temperature stability can be achieved. Especially at areaI-1, it is possible to achieve a surface acoustic wave device of broadpassband and good-enough temperature stability because a large couplingfactor can be obtained with a small TCV by a selection of the thicknessof the piezoelectric film.

It is to be noted that the primary temperature coefficient is hereinused as the temperature coefficient of SAW velocity, TCV. Even when atemperature vs. acoustic velocity curve takes the form of a quadraticcurve (the primary temperature coefficient is zero), the quadratic curvemay be approximated to a primary straight line by the method of leastsquares to calculate TCV. Specifically, TCV is found by dividing achange Δv of SAW velocity per unit temperature by SAW velocity v₀ at 0°C.

The cut direction of the substrate may be identified by means of x-raydiffraction.

The langasite single crystal used in the present invention is generallyrepresented by a chemical formula La₃ GasSiO₁₄, and known from Proc.IEEE International Frequency Control Sympo. vol. 1994, pp. 48-57(1994)as an example. In the present invention, the langasite single crystal isapplied to a surface acoustic wave device substrate. If, in this case,the cut direction of crystal and the propagation direction of surfaceacoustic waves are specifically selected, it is then possible to achievea surface acoustic wave device having such high characteristics asmentioned above. Langasite single crystals, if they are found by x-raydiffraction to be mainly composed of a langasite phase alone, may beused herein. In other words, the langasite single crystal used herein isnot always limited to that represented by the aforesaid chemicalformula. For instance, at least one part of each site for La, Ga, and Simay have been substituted by other element, or the number of oxygen maydepart from the aforesaid stoichiometric composition. In addition, thelangasite single crystal may contain inevitable impurities such as Al,Zr, Fe, Ce, Nd, Pt, and Ca. No particular limitation is imposed on howto produce the langasite single crystals; that is, they may be producedby ordinary single crystal growth processes, for instance, the CZprocess.

In embodiment 1, the preferable thickness for the piezoelectric film maybe determined depending on where (φ, θ, ψ) exist. More illustratively,preferable h/λ exists for each area, as already mentioned. Here, h isthe thickness of the piezoelectric film, λ is the wavelength of asurface acoustic wave, and h/λ is a value obtained by normalizing thethickness of the piezoelectric film by the wavelength of the surfaceacoustic wave. Generally at areas I-2 to I-9 included in the generalarea I, the larger the h/λ value is, the greater will be theelectromechanical coupling factor and the SAW velocity. At areas I-1,and I-10, the larger the h/λ value is, the lower will be the SAWvelocity. Thus, the range where a large-enough electromechanicalcoupling factor is obtained with a low-enough SAW velocity is selectedfor each area. The temperature coefficient of SAW velocity or TCV of asubstrate having a piezoelectric film is shifted in a negative directionwith an h/λ increase. When the substrate per se has a positive TCV,therefore, it is possible to achieve a TCV decrease by the provision ofthe piezoelectric film.

In each embodiment of the invention, no particular limitation is imposedon how to form the piezoelectric film; that is, the piezoelectric filmmay be formed by any desired process if it yields a piezoelectric filmhaving a piezoelectric axis oriented substantially perpendicularly withrespect to the surface of the substrate. Such a process, for instance,include a sputtering process, an ion plating process, and a CVD process.If these processes are used under appropriately preselectedfilm-formation conditions, it is possible to easily obtain apiezoelectric film having a piezoelectric or c-axis substantiallyperpendicularly with respect to the surface of the substrate.

Substrate size is not particularly critical in each embodiment of theinvention, and may generally be of the order of 1 to 20 mm in thepropagation direction of surface acoustic waves and of the order of 0.5to 5 mm in a direction perpendicular thereto. Substrate thickness isgenerally at least three times as large as the pitch of the interdigitalelectrodes (corresponding to the wavelength of a surface acoustic wave)formed on the substrate, and is usually of the order of 0.2 to 0.5 mm.However, it is to be noted that an experimental sample prepared for thepurpose of estimating the performance of the substrate may, in somecases, have a thickness exceeding the aforesaid upper limit of 0.5 mm.When, for instance, the pitch of an interdigital electrode is 320 μm forexperimental purposes, the thickness of the substrate is at least 0.96mm.

In the present invention, each of the interdigital electrodes 3 formedon the substrate 2 is a periodically striped thin film electrode forexciting, receiving, reflecting, and propagating surface acoustic waves.The interdigital electrode is patterned so as to achieve the aforesaidpreselected propagation direction of surface acoustic waves. Theinterdigital electrode may be formed as by evaporation or sputtering,using Au or Al. The electrode finger width of the interdigital electrodemay be determined depending on the frequency to which the surfaceacoustic wave device is applied, and may generally be of the order of 2to 10 μm at the frequency band to which the present invention ispreferably applied. The thickness of the interdigital electrode isusually of the order of 0.03 to 1.5 μm.

The surface acoustic wave device according to each embodiment ofinvention lends itself well to filters used at a frequency band of 10 to500 MHz in general, and 10 to 300 MHz in particular. The surfaceacoustic wave device of the present invention is useful for making asurface acoustic wave delay element small as well, because of its slowSAW velocity.

Embodiment 2

One exemplary architecture of the surface acoustic wave device accordingto embodiment 2 is shown in FIG. 16. This surface acoustic wave devicecomprises a substrate 2, a piezoelectric film 4 on the surface of thesubstrate 2, and a set of an input side interdigital electrode 3 and anoutput side interdigital electrode 3 formed on the surface of thepiezoelectric film 4.

When, in the surface acoustic wave device according to embodiment 2, thecut angle of the substrate out of the langasite single crystal and thepropagation direction of surface acoustic waves are represented in termsof Euler angles (φ, θ, ψ), φ, θ, and ψ exist in each of the areasmentioned above.

By selecting φ, θ and ψ from the general area II and providing apiezoelectric film of suitable thickness, it is possible to decrease theSAW velocity, increase the electromechanical coupling factor, anddecrease the temperature coefficient of SAW velocity or TCV. This inturn enables to reduce the size of a surface acoustic wave device, andimprove the passband width, and temperature stability of a surfaceacoustic wave device when it is used as a filter. In particular, it ispossible to achieve a surface acoustic wave filter best suited for usefor mobile communication terminal equipment operated at intermediatefrequencies. More illustratively, the temperature coefficient of SAWvelocity or TCV of the substrate can be in the range of -35 to 60 ppm/°C., the SAW velocity of the substrate can be up to 2,900 m/s, and thecoupling factor of the substrate can be 0.1% or greater. In some cases,much better characteristics may be obtained.

At the general area II, a broad band surface acoustic wave device can beachieved because a coupling factor of 0.4% or greater can be obtained.At areas II-1, and II-10, a broader band surface acoustic wave devicecan be achieved because a coupling factor of 0.8% or greater can beobtained.

At areas II-1, and II-10, a surface acoustic wave device of good-enoughtemperature stability can be achieved because TCV can be extremelyreduced. At areas II-1, and II-10, a surface acoustic wave device ofmuch broader passband and better temperature stability can be achievedbecause a great coupling factor can be obtained with a small TCV by aselection of the thickness of the piezoelectric film.

In embodiment 2, the preferable thickness for the piezoelectric film maybe determined depending on where (φ, θ, ψ) exist. More illustratively,preferable h/λ exists for each area. Here, h is the thickness of thepiezoelectric film, λ is the wavelength of a surface acoustic wave, andh/λ is a value obtained by normalizing the thickness of thepiezoelectric film by the wavelength of the surface acoustic wave.Generally at areas II-2 to II-9 included in the general area II, thelarger the h/λ value is, the greater will be the electromechanicalcoupling factor and the SAW velocity. At areas II-1, and II-10, thelarger the h/λ value is, the lower will be the SAW velocity. Thetemperature coefficient of SAW velocity or TCV of a substrate having apiezoelectric film is generally shifted in a negative direction with anh/λ increase. When the substrate per se has a positive TCV, therefore,it is possible to achieve a TCV decrease by the provision of thepiezoelectric film.

It is therefore preferable to select h/λ for each area in such a waythat the necessary characteristic or characteristics of the SAWvelocity, electromechanical coupling factor and TCV are well improved.In each area, there is a specific h/λ range where these characteristicsare practically satisfied as already mentioned.

Embodiment 3

One exemplary architecture of the surface acoustic wave device accordingto embodiment 3 is shown in FIG. 31. This surface acoustic wave devicecomprises a substrate 2, a set of an input side interdigital electrode 3and an output side interdigital electrode 3 on the surface of thesubstrate 2, and a piezoelectric film 4 provided to cover the substrate2 and the interdigital electrodes 3 and 3. In addition, an oppositeelectrode film 5 is formed on the surface of the piezoelectric film 4.

When, in the surface acoustic wave device according to embodiment 3, thecut angle of the substrate out of the langasite single crystal and thepropagation direction of surface acoustic waves are represented in termsof Euler angles (φ, θ, ψ), φ, θ, and ψ exist in each of the areasmentioned above.

By selecting φ, θ and ψ from the general area III and providing apiezoelectric film and an opposite electrode film, each of suitablethickness, it is possible to decrease the SAW velocity, increase theelectromechanical coupling factor, and decrease the temperaturecoefficient of SAW velocity or TCV. This in turn enables to reduce thesize of a surface acoustic wave device, and improve the passband width,and temperature stability of a surface acoustic wave device when it isused as a filter. In particular, it is possible to achieve a surfaceacoustic wave filter best suited for use for mobile communicationterminal equipment operated at intermediate frequencies. Moreillustratively, the temperature coefficient of SAW velocity or TCV ofthe substrate can be in the range of -35 to 60 ppm/° C., the SAWvelocity of the substrate can be up to 2,900 m/s, and the couplingfactor of the substrate can be 0.1% or greater. In some cases, muchbetter characteristics may be obtained.

At the general area III, the electromechanical coupling factor has twopeaks with respect to the thickness of the piezoelectric film. The peakof the electromechanical coupling factor on a thin piezoelectric filmside has a practically large-enough value of 0.13% or greater.Especially at area III-4, the peak value of the coupling factor is0.37%. The peak of the electromechanical coupling factor on a thickpiezoelectric film side, too, has a large-enough value of 0.15% orgreater. At this time, TCV shows an about 20 ppm/° C. improvement ascompared with that in the absence of a piezoelectric film.

In embodiment 3, the preferable thickness for the piezoelectric film maybe determined depending on where (φ, θ, ψ) exist. More illustratively,preferable h/λ exists for each area. Here, h is the thickness of thepiezoelectric film, λ is the wavelength of a surface acoustic wave, andh/λ is a value obtained by normalizing the thickness of thepiezoelectric film by the wavelength of the surface acoustic wave. Thelarger the h/λ value is, the lower will be the SAW velocity at areasIII-1 and III-10, and the higher will be the SAW velocity at areas III-2to III-9. As mentioned just above, at the general area III theelectromechanical coupling factor has two peaks with respect to h/λ.When the electromechanical coupling factor shows a peak on a large h/λside, the absolute value of TCV becomes small. The temperaturecoefficient of SAW velocity or TCV of a substrate having a piezoelectricfilm is shifted in a negative direction with an h/λ increase. When thesubstrate per se has a positive TCV, therefore, it is possible toachieve a TCV decrease by the provision of the piezoelectric film.

It is therefore preferable to select h/λ for each area in such a waythat the necessary characteristic or characteristics of the SAWvelocity, electromechanical coupling factor and TCV are well improved.In each area, there is a specific h/λ range where these characteristicsare practically satisfied as already mentioned.

The opposite electrode film 5 may be of such size as to cover the wholeof the piezoelectric film 4. Insofar as this embodiment is concerned,however, the opposite electrode film may be formed at least at a zoneopposite to the interdigital electrodes 3 on the input and output sides.The thickness of the opposite electrode film is preferably in the rangeof 0.03 to 0.1 μm. Too thin an opposite electrode film is not preferablebecause no continuous film is obtained or a electric potential in theplane of the film becomes inhomogeneous due to an electrical resistanceincrease. Too thick an opposite electrode film is again not preferablebecause there is an increase in the mass load of the opposite electrodefilm. The material, and formation of the opposite electrode film may bethe same as described in conjunction with the interdigital electrodes.The opposite electrode film is not always required to be grounded orotherwise connected; it may be placed in an electrically isolated state.

Embodiment 4

One exemplary architecture of the surface acoustic wave device accordingto embodiment 4 is shown in FIG. 46. This surface acoustic wave devicecomprises a substrate 2, an opposite electrode film 5 provided on thesurface of the substrate 2, a piezoelectric film 4 provided on thesurface of the opposite electrode film 5, and a set of an input sideinterdigital electrode 3 and an output side interdigital electrode 3provided on the surface of the piezoelectric film 4.

When, in the surface acoustic wave device according to embodiment 4, thecut angle of the substrate out of the langasite single crystal and thepropagation direction of surface acoustic waves are represented in termsof Euler angles (φ, θ, ψ), φ, θ, and ψ exist in each of the areasmentioned above.

By selecting φ, θ and ψ from the general area IV and providing apiezoelectric film and an opposite electrode film, each of suitablethickness, it is possible to decrease the SAW velocity, increase theelectromechanical coupling factor, and decrease the temperaturecoefficient of SAW velocity or TCV. This in turn enables to reduce thesize of a surface acoustic wave device, and improve the passband width,and temperature stability of a surface acoustic wave device when it isused as a filter. In particular, it is possible to achieve a surfaceacoustic wave filter best suited for use for mobile communicationterminal equipment operated at intermediate frequencies. Moreillustratively, the temperature coefficient of SAW velocity or TCV ofthe substrate can be in the range of -35 to 60 ppm/° C., the SAWvelocity of the substrate can be up to 2,900 m/s, and the couplingfactor of the substrate can be 0.1% or greater. In some cases, muchbetter characteristics may be obtained.

At the general area IV, a broad passband surface acoustic wave devicecan be achieved because a coupling factor of 0.2% or greater can beobtained. Especially at areas IV-1, and IV-10, a much broader passbandsurface acoustic wave device can be achieved because a coupling factorof 0.8% or greater can be obtained.

At areas IV-1, and IV-10, a surface acoustic wave device of good-enoughtemperature stability can be achieved because TCV can be extremelyreduced and, in some cases, can be reduced to zero. At areas IV-1, andIV-10, a surface acoustic wave device of much broader passband andbetter temperature stability can be achieved because a great couplingfactor can be obtained with a small TCV by a selection of the thicknessof the piezoelectric film.

In embodiment 4, the preferable thickness for the piezoelectric film maybe determined depending on where (φ, θ, ψ) exist, as already noted. Moreillustratively, preferable h/λ exists for each area. Here, h is thethickness of the piezoelectric film, X is the wavelength of a surfaceacoustic wave, and h/λ is a value obtained by normalizing the thicknessof the piezoelectric film by the wavelength of the surface acousticwave. Generally at areas IV-2 to IV-9 included in the general area IV,the larger the h/λ value is, the greater will be the electromechanicalcoupling factor and the SAW velocity. At areas IV-1, and IV-10, thelarger the h/λ value is, the lower will be the SAW velocity. Thetemperature coefficient of SAW velocity or TCV of a substrate having apiezoelectric film is generally shifted in a negative direction with anh/λ increase. When the substrate per se has a positive TCV, therefore,it is possible to achieve a TCV decrease by the provision of thepiezoelectric film.

It is therefore preferable to select h/λ for each area in such a waythat the necessary characteristic or characteristics of the SAWvelocity, electromechanical coupling factor and TCV are well improved.In each area, there is a specific h/λ range where these characteristicsare practically satisfied as already mentioned.

Insofar as this embodiment is concerned, the opposite electrode film 5may be formed at least at a zone opposite to the interdigital electrodes3 on the input and output sides. To make the piezoelectric film 4homogeneous, however, it is preferable to cover the whole of thesubstrate 2 with the opposite electrode film. The thickness of theopposite electrode film is preferably in the range of 0.03 to 0.1 μm.Too thin an opposite electrode film is not preferable because nocontinuous film is obtained or a electric potential in the plane of thefilm becomes inhomogeneous due to an electrical resistance increase. Toothick an opposite electrode film is again not preferable because thereis an increase in the mass load of the opposite electrode film. Thematerial, and formation of the opposite electrode film may be the sameas described in conjunction with the interdigital electrodes. Theopposite electrode film is not always required to be grounded orotherwise connected; it may be placed in an electrically isolated state.

EXAMPLE

The present invention will now be explained with reference to examples.

Example 1-1

(Embodiment 1)

A langasite single crystal was grown by the CZ process, and a substratewas cut out of this single crystal. A surface acoustic wave transducercomprising a set of an input side interdigital electrode and an outputside interdigital electrode was formed on the surface of the substrate,and a ZnO film was formed thereon by means of a magnetron sputteringprocess to fabricate a surface acoustic wave device. The interdigitalelectrodes, each being a normal type electrode of the same shape, wereformed of Al by evaporation, and had a thickness of 0.1 μm, an electrodefinger width d of 15 μm and an electrode pitch (4 d) of 60 μm(corresponding to the wavelength λ of a surface acoustic wave), with thenumber of electrode finger pairs being 40 and the aperture width ofelectrode fingers being 60λ (=3.6 mm). However, when output signals werefaint, interdigital electrodes with only the aperture width changed to100λ were used in place of the aforesaid interdigital electrodes. Inaddition, when the normalized thickness, h/λ, of the ZnO film exceeded0.4, the electrode pitch was halved to 30 μm, and the aperture width wascorrespondingly halved to 1.8 mm (=60λ).

In the instant example, φ and θ were 0° and 90° when the cut angle ofthe substrate in this device out of the langasite single crystal and thepropagation direction of surface acoustic waves thereon were expressedin terms of Euler angles (φ, θ and ψ). For ψ to determine the x-axis orthe propagation direction of surface acoustic waves, the values shown inFIGS. 2A to 10C were selected from within the general area I. Thethickness, h, of the ZnO film on the substrate was selected in such away that the aforesaid normalized thickness, h/λ, was between 0.05 and0.8. Changes in the SAW velocity, electromechanical coupling factor k²,and temperature coefficient of SAW velocity, TCV, were examined atvaried h/λ's in each propagation direction. The SAW velocity was foundfrom the center frequency of the filter, and the electromechanicalcoupling factor k² was determined from a two-terminal admittancemeasured of the surface acoustic wave transducer, using a well-knownSmith's equivalent circuit model. The results of the SAW velocity, k²,and TCV measured in each propagation direction are plotted in FIGS. 2Ato 10C.

At points with nothing plotted in each figure, surface acoustic wavesignals could not be detected. With consideration for the results of theelectromechanical coupling factor obtained, a possible explanation ofthis could be that when the propagation direction is in this range, theelectromechanical coupling factor becomes too small for efficientconversion of electrical signals to surface acoustic wave signals, andvice versa. As the normalized thickness, h/λ, of the ZnO film increased,the surface acoustic wave mode vanished, resulting in the generation ofbulk waves. For this reason, data obtained after the generation of bulkwaves are not plotted at all.

The graphs showing the SAW velocity, and k2 changes indicate that if thecut angle of the substrate from the crystal and the propagationdirection of surface acoustic waves lie in the general area I, the SAWvelocity can then be reduced to 2,900 m/s or lower. This is morefavorable for achieving a size reduction of a surface acoustic wavedevice, as compared with a conventional ST quartz crystal. It is alsofound that within the general area I an electromechanical couplingfactor of 0.1% or greater can be obtained. It is thus possible to obtaina much greater electromechanical coupling factor by a selection of thethickness of the ZnO film.

Referring further to the graphs showing the TCV changes, when thesubstrate per se has a positive TCV, in other words, when TCV ispositive at a normalized thickness, h/λ, of 0, it is found that TCV isshifted from a positive to a negative direction with an h/λ increase;improvements are made in the temperature properties. When the substrateitself has a negative TCV, on the other hand, TCV is greatly shifted toa negative side due to the provision of the ZnO film and an increase inits normalized thickness. Even in this case, the absolute value of TCVis not so great (of the order of 35 ppm/° C. or lower); it is found thatthe substrate can be much more improved in terms of temperaturestability over a conventional BGO substrate.

In what follows, each area will be explained at great length.

A device making use of area I-1 can have a very great electromechanicalcoupling factor of 0.76% when h/λ=0.6, as can be seen from FIGS. 2B, and2C. At this time, TCV=-26 ppm/° C.; good-enough temperature propertiesare obtained.

A device making use of area I-2 can have a great electromechanicalcoupling factor of 0.32% when h/λ=0.5, as can be seen from FIGS. 3B, and3C. At this time, TCV=9 ppm/° C.; very excellent temperature propertiesare obtained.

A device making use of area I-3 can have an electromechanical couplingfactor of 0.15% when h/λ=0.4, as can be seen from FIGS. 4B, and 4C. Atthis time, TCV=32 ppm/° C.; good-enough temperature properties areobtained.

A device making use of area I-4 can have a great electromechanicalcoupling factor of 0.19% when h/λ=0.4, as can be seen from FIGS. 5B, and5C. At this time, TCV=17 ppm/° C.; good-enough temperature propertiesare obtained.

A device making use of area I-5 can have a great electromechanicalcoupling factor of 0.25% when h/λ=0.35, as can be seen from FIGS. 6B,and 6C. At this time, TCV=16 ppm/° C.; good-enough temperatureproperties are obtained.

A device making use of area I-6 can have a very great electromechanicalcoupling factor of 0.61% when h/λ=0.35, as can be seen from FIGS. 7B,and 7C. At this time, TCV=17 ppm/° C.; good-enough temperatureproperties are obtained.

A device making use of area I-7 can have a very great electromechanicalcoupling factor of 0.72% when h/λ=0.35, as can be seen from FIGS. 8B,and 8C. At this time, TCV=19 ppm/° C.; good-enough temperatureproperties are obtained.

A device making use of area I-8 can have a very great electromechanicalcoupling factor of 0.53% when h/λ=0.35, as can be seen from FIGS. 9B,and 9C. At this time, TCV=33 ppm/° C.; good-enough temperatureproperties are obtained.

A device making use of area I-9 can have a very great electromechanicalcoupling factor of 0.63% when h/λ=0.5, as can be seen from FIGS. 10B,and 10C. At this time, TCV=12 ppm/° C.; good-enough temperatureproperties are obtained.

A device making use of area I-10 can have a very great electromechanicalcoupling factor of 0.96% when h/λ=0.55, as can be seen from FIGS. 11B,and 11C. At this time, TCV=-24 ppm/° C.; good-enough temperatureproperties are obtained.

Example 1-2

(Embodiment 1)

Surface acoustic wave devices were fabricated as in example 1-1 with theexception that φ and θ were 0° and 90°, respectively, and ψ to determinethe x-axis or the propagation direction of surface acoustic waves wasvaried at intervals of 2° between -80° and -66°. It is to be noted thatthese values for φ, θ and ψ were included in area I-1. For thesedevices, the TCV vs. h/λ (normalized thickness) relations were examined.The results are plotted in FIG. 12. Also, the k² vs. h/λ relations wereexamined. The results are plotted in FIG. 13.

From FIG. 12, on the one hand, it is seen that, at area I-1, a so-calledzero temperature property is obtained and the thickness of the ZnO filmat which the zero temperature property is obtained varies depending onthe propagation direction of surface acoustic waves. From FIG. 13, onthe other hand, it is seen that as the ZnO film becomes thick, theelectromechanical coupling factor tends to become large. Therefore, ifthe thickness of the ZnO film is determined in such a manner that thezero temperature property is obtained and if, at this time, thepropagation direction is selected in such a manner that a large-enoughelectromechanical coupling factor is obtained, it is then possible toachieve a surface acoustic wave device having the zero temperatureproperty and a large electromechanical coupling factor. If, forinstance, the propagation direction ψ is -70 and the normalizedthickness, h/λ, of ZnO is 0.35, it is then possible to achieve a smallsize yet broad passband surface acoustic wave device having excellenttemperature properties because TCV is substantially reduced to zero anda very large k² of 0.32% is obtained.

Example 1-3

(Embodiment 1)

Surface acoustic wave devices were fabricated as in example 1-1 with theexception that φ and θ were 0° and 90°, respectively, and ψ to determinethe x-axis or the propagation direction of surface acoustic waves wasvaried at intervals of 2° between 66° and 80°. It is to be noted thatthese values for φ, θ and ψ were included in area I-10. For thesedevices, the TCV vs. h/λ (normalized thickness) relations were examined.The results are plotted in FIG. 14. Also, the k² vs. h/λ relations wereexamined. The results are plotted in FIG. 15.

From FIG. 14, on the one hand, it is seen that, at area I-10, the zerotemperature property is obtained and the thickness of the ZnO film atwhich the zero temperature property is obtained varies depending on thepropagation direction of surface acoustic waves. From FIG. 15, on theother hand, it is seen that as the ZnO film becomes thick, theelectromechanical coupling factor tends to become large. Therefore, ifthe thickness of the ZnO film is determined in such a manner that alarge-enough electromechanical coupling factor is obtained and if, atthis time, the propagation direction is selected in such a manner thatthe zero temperature property is obtained, it is then possible toachieve a surface acoustic wave device having the zero temperatureproperty and a large electromechanical coupling factor. If, forinstance, the propagation direction ψ is 70° and the normalizedthickness, h/═, of ZnO is 0.35, it is then possible to achieve a smallsize yet broad passband surface acoustic wave device having excellenttemperature properties because TCV is substantially reduced to zero anda very large k² of 0.6% is obtained.

Example 2-1

(Embodiment 2)

A langasite single crystal was grown by the CZ process, and a 0.35 mmthick substrate was cut out of this single crystal. A ZnO film wasformed on the surface of the substrate by a magnetron sputteringprocess, and a surface acoustic wave transducer comprising a set of aninput side interdigital electrode and an output side interdigitalelectrode was formed on the surface of the ZnO film to fabricate asurface acoustic wave device. The interdigital electrodes, each being anormal type electrode of the same shape, were formed of Al byevaporation, and had a thickness of 0.1 μm, an electrode finger width dof 15 μm and an electrode pitch (4 d) of 60 μm (corresponding to thewavelength λ of a surface acoustic wave), with the number of electrodefinger pairs being 40 and the aperture width of electrode fingers being60λ (=3.6 mm). However, when output signals were faint, interdigitalelectrodes with only the aperture width changed to 100λ were used inplace of the aforesaid interdigital electrodes. In addition, when thenormalized thickness, h/λ, of the ZnO film exceeded 0.4, the electrodepitch was halved to 30 μm, and the aperture width was correspondinglyhalved to 1.8 mm (=60λ).

In the instant example, φ and θ were 0° and 90° when the cut angle ofthe substrate in this device out of the langasite single crystal and thepropagation direction of surface acoustic waves thereon were expressedin terms of Euler angles (φ, θ and ψ). For ψ to determine the x-axis orthe propagation direction of surface acoustic waves, the values shown inFIGS. 17A to 26C were selected from within the general area II. Thethickness, h, of the ZnO film on the substrate was selected in such away that the aforesaid normalized thickness, h/λ, was between 0.05 and0.8. For the purpose of comparison, a surface acoustic wave device withh/λ=0, i.e., with no ZnO film formed thereon, was prepared. Changes inthe SAW velocity, electromechanical coupling factor k², and temperaturecoefficient of SAW velocity, TCV, were examined at varied h/λ's in eachpropagation direction. The SAW velocity was found from the centerfrequency of the filter, and the electromechanical coupling factor k²was determined from a two-terminal admittance measured of the surfaceacoustic wave transducer, using a well-known Smith's equivalent circuitmodel. The results of the SAW velocity, k², and TCV measured in eachpropagation direction are plotted in FIGS. 17A to 26C.

As the normalized thickness, h/λ, of the ZnO film increased, the surfaceacoustic wave mode vanished, resulting in the generation of bulk waves.For this reason, data obtained after the generation of bulk waves arenot plotted at all.

The graphs showing the SAW velocity, and k² changes indicate that if thecut angle of the substrate from the crystal and the propagationdirection of surface acoustic waves lie in the general area II, the SAWvelocity can then be reduced to 2,900 m/s or lower. This is morefavorable for achieving a size reduction of a surface acoustic wavedevice, as compared with a conventional ST quartz crystal. It is alsofound that within the general area II an electromechanical couplingfactor of 0.1% or greater can be obtained. It is thus possible to obtaina much greater electromechanical coupling factor by a selection of thethickness of the ZnO.

Referring further to the graphs showing the TCV changes, when thesubstrate per se has a positive TCV, in other words, when TCV ispositive at a normalized thickness, h/λ, of 0, it is found that TCV isshifted from a positive to a negative direction with an h/λ increase;improvements are made in the temperature properties. When the substrateitself has a Inegative TCV, on the other hand, TCV is greatly shifted toa negative side due to the provision of the ZnO film and an increase inits normalized thickness. Even in this case, the absolute value of TCVis not so great (of the order of 35 ppm/° C. or lower); it is found thatthe substrate can be much more improved in terms of temperaturestability over a conventional BGO substrate.

In what follows, each area will be explained at great length.

A device making use of area II-1 can have a very great electromechanicalcoupling factor of 0.88% when h/λ=0.8, as can be seen from FIGS. 17B,and 17C. At this time, TCV=-30 ppm/° C.; good-enough temperatureproperties are obtained. A device making use of area II-2 can have agreat electromechanical coupling factor of 0.6% when h/λ=0.55, as can beseen from FIGS. 18B, and 18C. At this time, TCV=9 ppm/° C.; veryexcellent temperature properties are obtained.

A device making use of area II-3 can have an electromechanical couplingfactor of 0.44% when h/λ=0.35, as can be seen from FIGS. 19B, and 19C.At this time, TCV=29 ppm/° C.; good-enough temperature properties areobtained.

A device making use of area II-4 can have a great electromechanicalcoupling factor of 0.56% when h/λ=0.4, as can be seen from FIGS. 20B,and 20C. At this time, TCV=17 ppm/° C.; good-enough temperatureproperties are obtained.

A device making use of area II-5 can have a very great electromechanicalcoupling factor of 0.53% when h/λ=0.35, as can be seen from FIGS. 21B,and 21C. At this time, TCV=15 ppm/° C.; good-enough temperatureproperties are obtained.

A device making use of area II-6 can have a very great electromechanicalcoupling factor of 0.59% when h/λ=0.3, as can be seen from FIGS. 22B,and 22C. At this time, TCV=16 ppm/° C.; very excellent temperatureproperties are obtained.

A device making use of area II-7 can have a very great electromechanicalcoupling factor of 0.63% when h/λ=0.35, as can be seen from FIGS. 23B,and 23C. At this time, TCV=19 ppm/° C.; good-enough temperatureproperties are obtained.

A device making use of area II-8 can have a very great electromechanicalcoupling factor of 0.51% when h/λ=0.3, as can be seen from FIGS. 24B,and 24C. At this time, TCV=32 ppm/° C.; good-enough temperatureproperties are obtained.

A device making use of area II-9 can have a very great electromechanicalcoupling factor of 0.59% when h/λ=0.55, as can be seen from FIGS. 25B,and 25C. At this time, TCV=11 ppm/° C.; good-enough temperatureproperties are obtained.

A device making use of area II-10 can have a very greatelectromechanical coupling factor of 0.86% when h/λ=0.75, as can be seenfrom FIGS. 26B, and 26C. At this time, TCV=-30 ppm/° C.; good-enoughtemperature properties are obtained.

Example 2-2

(Embodiment 2)

Surface acoustic wave devices were fabricated as in example 2-1 with theexception that φ and θ were 0° and 90°, respectively, and ψ to determinethe x-axis or the propagation direction of surface acoustic waves wasvaried at intervals of 2° between -80° and -66°. It is to be noted thatthese values for φ, θ and ψ were included in area II-1. For thesedevices, the TCV vs. h/λ (normalized thickness) relations were examined.The results are plotted in FIG. 27. Also, the k² vs. h/λ relations wereexamined. The results are plotted in FIG. 28.

From FIG. 27, on the one hand, it is seen that, at area II-1, the zerotemperature property is obtained and the thickness of the ZnO film atwhich the zero temperature property is obtained varies depending on thepropagation direction of surface acoustic waves. From FIG. 28, on theother hand, it is seen that as the ZnO film becomes thick, theelectromechanical coupling factor tends to become large. Therefore, ifthe thickness of the ZnO film is determined in such a manner that alarge-enough electromechanical coupling factor is obtained and if, atthis time, the propagation direction is selected in such a manner thatthe zero temperature property is obtained, it is then possible toachieve a surface acoustic wave device having the zero temperatureproperty and a large electromechanical coupling factor. If, forinstance, the propagation direction ψ is -70° and the normalizedthickness, h/λ, of ZnO is 0.35, it is then possible to achieve a smallsize yet broad passband surface acoustic wave device having excellenttemperature properties because TCV is substantially reduced to zero anda very large k² of 0.51% is obtained.

Example 2-3

(Embodiment 2)

Surface acoustic wave devices were fabricated as in example 2-1 with theexception that φ and θ were 0° and 90°, respectively and ψ to determinethe x-axis or the propagation direction of surface acoustic waves wasvaried at intervals of 2° between 66° and 80°. It is to be noted thatthese values for φ, θ and ψ0 were included in area II-10. For thesedevices, the TCV vs. h/λ (normalized thickness) relations were examined.The results are plotted in FIG. 29. Also, the k² vs. h/λ relations wereexamined. The results are plotted in FIG. 30.

From FIG. 29, on the one hand, it is seen that, at area II-10, the zerotemperature property can be obtained and the thickness of the ZnO filmat which the zero temperature property is obtained varies depending onthe propagation direction of surface acoustic waves. From FIG. 30, onthe other hand, it is seen that as the ZnO film becomes thick, theelectromechanical coupling factor tends to become large. Therefore, ifthe thickness of the ZnO film is determined in such a manner that alarge-enough electromechanical coupling factor is obtained and if, atthis time, the propagation direction is selected in such a manner thatthe zero temperature property is obtained, it is then possible toachieve a surface acoustic wave device having the zero temperatureproperty and a large electromechanical coupling factor. If, forinstance, the propagation direction ψ is 70° and the normalizedthickness, h/λ, of ZnO is 0.35, it is then possible to achieve a smallsize yet broad passband surface acoustic wave device having excellenttemperature properties because TCV is substantially reduced to zero anda very large k² of 0.56% is obtained.

Example 3-1

(Embodiment 3)

A langasite single crystal was grown by the CZ process, and a 0.35 mmthick substrate was cut out of this single crystal. A surface acousticwave transducer comprising a set of an input side interdigital electrodeand an output side interdigital electrode was formed on the surface ofthe substrate, and a ZnO film was formed thereon by means of a magnetronsputtering process. Then, an opposite electrode film was formed on theZnO film to fabricate a surface acoustic wave device. The interdigitalelectrodes and opposite electrode films were formed of Al byevaporation. The interdigital electrodes, each being a normal typeelectrode of the same shape, had a thickness of 0.1 μm, an electrodefinger width d of 15 μm and an electrode pitch (4 d=λ) of 60 μm), withthe number of electrode fingers pairs being 40 and the aperture width ofelectrode fingers being 60λ (=3.6 mm). However, when output signals werefaint, interdigital electrodes with only the aperture width changed to100λ were used in place of the aforesaid interdigital electrodes. Inaddition, when the normalized thickness, h/λ, of the ZnO film exceeded0.4, the electrode pitch was halved to 30 μm, and the aperture width wascorrespondingly halved to 1.8 mm (=60w). The opposite electrode film hada thickness of 0.1 μm.

The surface acoustic wave device according to embodiment 3, because ofhaving the structure wherein the interdigital electrodes oppose to theopposite electrode film with the ZnO film interleaved between them, isunlikely to operate as the thickness of the ZnO film is close to zero.This is because short-circuiting occurs between the interdigitalelectrodes and the opposite electrode film. In the instant example,therefore, the minimum value for the normalized thickness, h/λ, of theZnO film was preset at 0.005. For a device having a normalized thicknessof 0.005, use was made of interdigital electrodes having a thickness of0.1 μm, an electrode pitch (=λ) of 320 μm, 20 electrode finger pairs,and a aperture width of 5 mm, an opposite electrode film having athickness of 0.07 μm, and a substrate having a thickness of 1 mm.

In the instant example, φ and θ were 0° and 90° when the cut angle ofthe substrate in this device out of the langasite single crystal and thepropagation direction of surface acoustic waves thereon were expressedin terms of Euler angles (φ, θ and ψ). For ψ to determine the x-axis orthe propagation direction of surface acoustic waves, the values shown inFIGS. 32A to 41C were selected from within the general area III. Thethickness, h, of the ZnO film on the substrate was selected in such away that the aforesaid normalized thickness, h/λ, was between 0.005 and0.8. Changes in the SAW velocity, electromechanical coupling factor k²,and temperature coefficient of SAW velocity, TCV, were examined atvaried h/λ's in each propagation direction. In this regard, it is to benoted that the value of λ varies depending on the value of h/λ. The SAWvelocity was found from the center frequency of the filter, and theelectromechanical coupling factor k² was determined from a two-terminaladmittance measured of the surface acoustic wave transducer, using awell-known Smith's equivalent circuit model. The results of the SAWvelocity, k², and TCV measured in each propagation direction are plottedin FIGS. 32A to 41C.

As the normalized thickness, h/α, of the ZnO film increased, the surfaceacoustic wave mode vanished, resulting in the generation of bulk waves.For this reason, data obtained after the generation of bulk waves arenot plotted at all.

The graphs showing the SAW velocity, and k2 changes indicate that if thecut angle of the substrate from the crystal and the propagationdirection of surface acoustic waves lie in the general area III, the SAWvelocity can then be reduced to 2,900 m/s or lower. This is morefavorable for achieving a size reduction of a surface acoustic wavedevice, as compared with a conventional ST quartz crystal. It is alsofound that within the general area III an electro-mechanical couplingfactor of 0.1% or greater can be obtained. It is thus possible to obtaina much greater electromechanical coupling factor by a selection of thethickness of the ZnO film.

Referring further to the graphs showing the TCV changes, when thesubstrate per se has a positive TCV, in other words, when TCV ispositive upon the normalized thickness, h/λ, being close to 0, it isfound that TCV is shifted from a positive to a negative direction withan h/λ increase; improvements are made in the temperature properties.When the substrate itself has a negative TCV, on the other hand, TCV isgreatly shifted to a negative side due to the provision of the ZnO filmand an increase in its normalized thickness. Even in this case, theabsolute value of TCV is not so great (of the order of 35 ppm/° C. orlower); it is found that the substrate can be much more improved interms of temperature stability over a conventional BGO substrate.

In what follows, each area will be explained at great length.

A device making use of area III-1 can have electro-mechanical couplingfactor peaks at h/λ=0.05 and h/λ=0.65, as can be seen from FIG. 32B. Ath/λ=0.05 an electromechanical coupling factor of 0.22% is obtained, andat h/λ=0.65 a very great electromechanical coupling factor of 0.71% isobtained. At this time, TCV=-3 ppm/° C. in the former case and TCV=-27ppm/° C. in the latter case, as can be seen from FIG. 32C; good-enoughtemperature properties are obtained.

A device making use of area III-2 can have electromechanical couplingfactor peaks at h/λ=0.05 and h/λ=0.6, as can be seen from FIG. 33B. Ath/λ=0.05 an electromechanical coupling factor of 0.2% is obtained, andat h/λ=0.6 a great electromechanical coupling factor of 0.3% isobtained. At this time, TCV=29 ppm/° C. in the former case and TCV=5ppm/° C. in the latter case, as can be seen from FIG. 33C; good-enoughtemperature properties are obtained.

A device making use of area III-3 can have electromechanical couplingfactor peaks at h/λ=0.05 and h/λ=0.45, as can be seen from FIG. 34B. Ath/λ=0.05 an electromechanical coupling factor of 0.29% is obtained, andat h/λ=0.45 a great electromechanical coupling factor of 0.12% isobtained. At this time, TCV=40 ppm/° C. in the former case and TCV=31ppm/° C. in the latter case, as can be seen from FIG. 34C; good-enoughtemperature properties are obtained.

A device making use of area III-4 can have electromechanical couplingfactor peaks at h/λ=0.05 and h/λ=0.45, as can be seen from FIG. 35B. Ath/λ=0.05 an electromechanical coupling factor of 0.37% is obtained, andat h/λ=0.45 a great electromechanical coupling factor of 0.2% isobtained. At this time, TCV=32 ppm/° C. in the former case and TCV=15ppm/° C. in the latter case, as can be seen from FIG. 35C; good-enoughtemperature properties are obtained.

A device making use of area III-5 can have electromechanical couplingfactor peaks at h/λ=0.05 and h/λ=0.4, as can be seen from FIG. 36B. Ath/λ=0.05 an electromechanical coupling factor of 0.36% is obtained, andat h/λ 0.4 a great electromechanical coupling factor of 0.2% isobtained. At this time, TCV=27 ppm/° C. in the former case and TCV=14ppm/° C. in the latter case, as can be seen from FIG. 36C; good-enoughtemperature properties are obtained.

A device making use of area III-6 can have electromechanical couplingfactor peaks at h/λ=0.05 and h/λ=0.4, as can be seen from FIG. 37B. Ath/λ=0.05 an electromechanical coupling factor of 0.29% is obtained, andat h/λ=0.4 a great electromechanical coupling factor of 0.5% isobtained. At this time, TCV=25 ppm/° C. in the former case and TCV=16ppm/° C. in the latter case, as can be seen from FIG. 37C; good-enoughtemperature properties are obtained.

A device making use of area III-7 can have electromechanical couplingfactor peaks at h/λ=0.05 and h/λ=0.45, as can be seen from FIG. 38B. Ath/λ=0.05 an electromechanical coupling factor of 0.24% is obtained, andat h/λ=0.45 a very great electromechanical coupling factor of 0.65% isobtained. At this time, TCV=31 ppm/° C. in the former case and TCV=18ppm/° C. in the latter case, as can be seen from FIG. 38C; good-enoughtemperature properties are obtained.

A device making use of area III-8 can have electromechanical couplingfactor peaks at h/λ=0.05 and h/λ=0.4, as can be seen from FIG. 39B. Ath/λ=0.05 an electromechanical coupling factor of 0.18% is obtained, andat h/λ=0.4 a great electromechanical coupling factor of 0.45% isobtained. At this time, TCV=39 ppm/° C. in the former case and TCV=31ppm/° C. in the latter case, as can be seen from FIG. 39C; good-enoughtemperature properties are obtained.

A device making use of area III-9 can have electromechanical couplingfactor peaks at h/λ=0.05 and h/λ=0.55, as can be seen from FIG. 40B. Ath/λ=0.05 an electromechanical coupling factor of 0.13% is obtained, andat h/λ=0.55 a great electromechanical coupling factor of 0.6% isobtained. At this time, TCV=29 ppm/° C. in the former case and TCV=7ppm/° C. in the latter case, as can be seen from FIG. 40C; good-enoughtemperature properties are obtained.

A device making use of area III-10 can have electromechanical couplingfactor peaks at h/λ=0.05 and h/λ=0.6, as can be seen from FIG. 41B. Ath/λ=0.05 an electromechanical coupling factor of 0.14% is obtained, andat h/λ=0.6 a great electromechanical coupling factor of 0.89% isobtained. At this time, TCV=-2 ppm/° C. in the former case and TCV=-27ppm/° C. in the latter case, as can be seen from FIG. 41C; good-enoughtemperature properties are obtained.

Example 3-2

(Embodiment 3)

Surface acoustic wave devices were fabricated as in example 3-1 with theexception that φ and θ were 0° and 90°, respectively, and ψ to determinethe x-axis or the propagation direction of surface acoustic waves wasvaried at intervals of 2° between -80° and -66°. It is to be noted thatthese values for φ, θ and ψ were included in area III-1. For thesedevices, the TCV vs. h/λ (normalized thickness) relations were examined.The results are plotted in FIG. 42. Also, the k² vs. h/λ relations wereexamined. The results are plotted in FIG. 43.

From FIG. 42, on the one hand, it is seen that, at area III-1, the zerotemperature property can be obtained and the thickness of the ZnO filmat which the zero temperature property is obtained varies depending onthe propagation direction of surface acoustic waves. From FIG. 43, onthe other hand, it is seen that as the electromechanical coupling factorhas two peaks, and that as the ZnO film becomes thicker, the peakbecomes larger. Therefore, if the thickness of the ZnO film isdetermined in such a manner that a large-enough electromechanicalcoupling factor is obtained and if, at this time, the propagationdirection is selected in such a manner that the zero temperatureproperty is obtained, it is then possible to achieve a surface acousticwave device having the zero temperature property and a largeelectromechanical coupling factor. If, for instance, the propagationdirection ψ is -78° and the normalized thickness, h/λ, of ZnO is 0.05,it is then possible to achieve a small size yet broad passband surfaceacoustic wave device having excellent temperature properties because TCVis substantially reduced to zero and a practical-enough k² of 0.21% isobtained.

Example 3-3

(Embodiment 3)

Surface acoustic wave devices were fabricated as in example 3-1 with theexception that φ and θ were 0° and 90°, respectively, and ψ to determinethe x-axis or the propagation direction of surface acoustic waves wasvaried at intervals of 2° between 66° and 80°. It is to be noted thatthese values for φ, θ and ψ were included in area I11-10. For thesedevices, the TCV vs. h/λ (normalized thickness) relations were examined.The results are plotted in FIG. 44. Also, the k² vs. h/λ relations wereexamined. The results are plotted in FIG. 45.

From FIG. 44, on the one hand, it is seen that, at area III-10, the zerotemperature property can be obtained and the thickness of the ZnO filmat which the zero temperature property is obtained varies depending onthe propagation direction of surface acoustic waves. From FIG. 45, onthe other hand, it is seen that as the ZnO film becomes thick, theelectromechanical coupling factor tends to become large. Therefore, ifthe thickness of the ZnO film is determined in such a manner that alarge-enough electromechanical coupling factor is obtained and if, atthis time, the propagation direction is selected in such a manner thatthe zero temperature property is obtained, it is then possible toachieve a surface acoustic wave device having the zero temperatureproperty and a large electromechanical coupling factor. If, forinstance, the propagation direction ψ is 70° and the normalizedthickness, h/λ, of ZnO is 0.35, it is then possible to achieve a smallsize yet broad passband surface acoustic wave device having excellenttemperature properties because TCV is substantially reduced to zero anda large k² of 0.44% is obtained.

Example 4-1

(Embodiment 4)

A langasite single crystal was grown by the CZ process, and a 0.35 mmthick substrate was cut out of this single crystal. An oppositeelectrode film was formed on the surface of the substrate. Then, a ZnOfilm was formed on the surface of the opposite electrode film by amagnetron sputtering process, and a surface acoustic wave transducercomprising a set of an input side interdigital electrode and an outputside interdigital electrode was finally formed on the surface of the ZnOfilm to fabricate a surface acoustic wave device. The interdigitalelectrodes and opposite electrode film were formed by the evaporation ofAl. The interdigital electrodes, each being a normal type electrode ofthe same shape, had a thickness of 0.1 μm, an electrode finger width dof 15 μm and an electrode pitch (=4 d=λ) of 60 μm, with the number ofelectrode finger pairs being 40 and the aperture width of electrodefingers being 60λ (=3.6 mm). However, when output signals were faint,interdigital electrodes with only the aperture width changed to 100λwere used in place of the aforesaid interdigital electrodes. Inaddition, when the normalized thickness, h/λ, of the ZnO film exceeded0.4, the electrode pitch (=λ) was halved to 30 μm, and the aperturewidth was correspondingly halved to 1.8 mm (=60λ).

The surface acoustic wave device according to embodiment 4, because ofhaving the structure wherein the interdigital electrodes oppose to theopposite electrode film with the ZnO film interleaved between them, isunlikely to operate as the thickness of the ZnO film is close to zero.This is because short-circuiting occurs between the interdigitalelectrodes and the opposite electrode film. In the instant example,therefore, the minimum value for the normalized thickness, h/λ, of theZnO film was preset at 0.005. For a device having a normalized thicknessof 0.005, use was made of interdigital electrodes having a thickness of0.1 λm, an electrode pitch of 320 μm, 20 electrode finger pairs, and aaperture width of 5 mm, an opposite electrode film having a thickness of0.07 μm, and a substrate having a thickness of 1 mm.

In the instant example, φ and θ were 0° and 90° when the cut angle ofthe substrate in this device out of the langasite single crystal and thepropagation direction of surface acoustic waves thereon were expressedin terms of Euler angles (φ, θ and ψ). For ψ to determine the x-axis orthe propagation direction of surface acoustic waves, the values shown inFIGS. 47A to 56C were selected from within the general area IV. Thethickness, h, of the ZnO film on the substrate was selected in such away that the aforesaid normalized thickness, h/λ, was between 0.05 and0.8. Changes in the SAW velocity, electromechanical coupling factor k²,and temperature coefficient of SAW velocity, TCV, were examined atvaried h/λ's in each propagation direction. In this regard, it is to benoted that the value of x varies depending on the value of h/λ, asalready described. The SAW velocity was found from the center frequencyof the filter, and the electromechanical coupling factor k² wasdetermined from a two-terminal admittance measured of the surfaceacoustic wave transducer, using a well-known Smith's equivalent circuitmodel. The results of the SAW velocity, k², and TCV measured in eachpropagation direction are plotted in FIGS. 47A to 56C.

As the normalized thickness, h/λ, of the ZnO film increased, the surfaceacoustic wave mode vanished, resulting in the generation of bulk waves.For this reason, data obtained after the generation of bulk waves arenot plotted at all.

The graphs showing the SAW velocity, and k² changes indicate that if thecut angle of the substrate from the crystal and the propagationdirection of surface acoustic waves lie in the general area IV, the SAWvelocity can then be reduced to 2,900 m/s or lower. This is morefavorable for achieving a size reduction of a surface acoustic wavedevice, as compared with a conventional ST quartz crystal. It is alsofound that within the general area IV an electromechanical couplingfactor of 0.1% or greater can be obtained. It is thus possible to obtaina much greater electromechanical coupling factor by a selection of thethickness of the ZnO.

Referring further to the graphs showing the TCV changes, when thesubstrate per se has a positive TCV, in other words, when TCV ispositive upon the normalized thickness, h/λ, being close to 0, it isfound that TCV is shifted from a positive to a negative direction withan h/λ increase; improvements are made in the temperature properties.When the substrate itself has a negative TCV, on the other hand, TCV isgreatly shifted to a negative side due to the provision of the ZnO filmand an increase in its normalized thickness. Even in this case, theabsolute value of TCV is not so great (of the order of 35 ppm/° C. orlower); it is found that the substrate can be much more improved interms of temperature stability over a conventional BGO substrate.

In what follows, each area will be explained at great length.

A device making use of area IV-1 can have a very great electromechanicalcoupling factor of 0.88% when h/λ=0.8, as can be seen from FIGS. 47B,and 47C. At this time, TCV=-31 ppm/° C.; good-enough temperatureproperties are obtained.

A device making use of area IV-2 can have a great electromechanicalcoupling factor of 0.6% when h/λ=0.6, as can be seen from FIGS. 48B, and48C. At this time, TCV=6 ppm/° C.; very excellent temperature propertiesare obtained.

A device making use of area IV-3 can have an electromechanical couplingfactor of 0.39% when h/λ=0.4, as can be seen from FIGS. 49B, and 49C. Atthis time, TCV=29 ppm/° C.; good-enough temperature properties areobtained.

A device making use of area IV-4 can have a great electromechanicalcoupling factor of 0.52% when h/λ=0.45, as can be seen from FIGS. 50B,and 50C. At this time, TCV=17 ppm/° C.; good-enough temperatureproperties are obtained.

A device making use of area IV-5 can have a very great electromechanicalcoupling factor of 0.46% when h/λ=0.4, as can be seen from FIGS. 51B,and 51C. At this time, TCV=15 ppm/° C.; good-enough temperatureproperties are obtained.

A device making use of area IV-6 can have a very great electromechanicalcoupling factor of 0.46% when h/λ=0.4, as can be seen from FIGS. 52B,and 52C. At this time, TCV=15 ppm/° C.; good-enough temperatureproperties are obtained.

A device making use of area IV-7 can have a very great electromechanicalcoupling factor of 0.52% when h/λ=0.45, as can be seen from FIGS. 53B,and 53C. At this time, TCV=17 ppm/° C.; good-enough temperatureproperties are obtained.

A device making use of area IV-8 can have a very great electromechanicalcoupling factor of 0.39% when h/λ=0.4, as can be seen from FIGS. 54B,and 54C. At this time, TCV=29 ppm/° C.; good-enough temperatureproperties are obtained.

A device making use of area IV-9 can have a very great electromechanicalcoupling factor of 0.6% when h/λ=0.6, as can be seen from FIGS. 55B, and55C. At this time, TCV=6 ppm/° C.; good-enough temperature propertiesare obtained.

A device making use of area IV-10 can have a very greatelectromechanical coupling factor of 0.88% when h/λ=0.8, as can be seenfrom FIGS. 56B, and 56C. At this time, TCV=-32 ppm/° C.; good-enoughtemperature properties are obtained.

Example 4-2

(Embodiment 4)

Surface acoustic wave devices were fabricated as in example 4-1 with theexception that φ and θ were 0° and 90°, respectively and ψ to determinethe x-axis or the propagation direction of surface acoustic waves wasvaried at intervals of 2° between -80° and -66°. It is to be noted thatthese values for φ, θ and ψ were included in area IV-1. For thesedevices, the TCV vs. h/λ (normalized thickness) relations were examined.The results are plotted in FIG. 57. Also, the k² vs. h/λ relations wereexamined. The results are plotted in FIG. 58.

From FIG. 57, on the one hand, it is seen that, at area IV-1, the zerotemperature property can be obtained and the thickness of the ZnO filmat which the zero temperature property is obtained varies depending onthe propagation direction of surface acoustic waves. From FIG. 58, onthe other hand, it is seen that as the ZnO film becomes thick, theelectromechanical coupling factor tends to become large. Therefore, ifthe thickness of the ZnO film is determined in such a manner that alarge-enough electromechanical coupling factor is obtained and if, atthis time, the propagation direction is selected in such a manner thatthe zero temperature property is obtained, it is then possible toachieve a surface acoustic wave device having the zero temperatureproperty and a large electromechanical coupling factor. If, forinstance, the propagation direction ψ is -70 and the normalizedthickness, h/λ, of ZnO is 0.35, it is then possible to achieve a smallsize yet broad passband surface acoustic wave device having excellenttemperature properties because TCV is substantially reduced to zero anda very large k² of 0.42% is obtained.

Example 4-3

(Embodiment 4)

Surface acoustic wave devices were fabricated as in example 4-1 with theexception that φ and θ were 0° and 90°, respectively and ψ to determinethe x-axis or the propagation direction of surface acoustic waves wasvaried at intervals of 2° between 66° and 80°. It is to be noted thatthese values for φ, θ and ψ were included in area IV-10. For thesedevices, the TCV vs. h/λ (normalized thickness) relations were examined.The results are plotted in FIG. 59. Also, the k² vs. h/λ relations wereexamined. The results are plotted in FIG. 60.

From FIG. 59, on the one hand, it is seen that, at area IV-10, the zerotemperature property can be obtained and the thickness of the ZnO filmat which the zero temperature property is obtained varies depending onthe propagation direction of surface acoustic waves. From FIG. 60, onthe other hand, it is seen that as the ZnO film becomes thick, theelectromechanical coupling factor tends to become large. Therefore, ifthe thickness of the ZnO film is determined in such a manner that alarge-enough electromechanical coupling factor is obtained and if, atthis time, the propagation direction is selected in such a manner thatthe zero temperature property is obtained, it is then possible toachieve a surface acoustic wave device having the zero temperatureproperty and a large electromechanical coupling factor. If, forinstance, the propagation direction ψ is 70° and the normalizedthickness, h/λ, of ZnO is 0.35, it is then possible to achieve a smallsize yet broad passband surface acoustic wave device having excellenttemperature properties because TCV is substantially reduced to zero anda very large k² of 0.42% is obtained.

The results of the examples clarify the advantages of the invention.

What we claim is:
 1. A surface acoustic wave device, comprising:a substrate, an interdigital electrode on a surface of said substrate, and a piezoelectric film configured to cover said surface of said substrate and a surface of said interdigital electrode, wherein: said substrate is a langasite single crystal belonging to a point group 32, and when a cut angle of said substrate cut out of the langasite single crystal and a propagation direction of surface acoustic waves on said substrate are represented in terms of Euler angles (φ, θ, ψ), where -5°≦φ≦5° 85°≦θ≦95° -90°≦ψ<-70°, andsaid piezoelectric film is a c-axis oriented ZnO film that satisfies:

    h/λ=0.2 to 0.8

where h is a thickness of said ZnO film, and λ is a wavelength of a surface acoustic wave.
 2. A surface acoustic wave device comprising:a substrate, an interdigital electrode on a surface of said substrate, and a piezoelectric film configured to cover said surface of said substrate and a surface of said interdigital electrode, wherein: said substrate is a langasite single crystal belonging to a point group 32, and when a cut angle of said substrate cut out of the langasite single crystal and a propagation direction of surface acoustic waves on said substrate are represented in terms of Euler angles (φ, θ, ψ), where -5°≦φ≦5° 85°>θ≦95° -70°≦ψ<-50°, andsaid piezoelectric film is a c-axis oriented ZnO film that satisfies:

    h/λ=0.25 to 0.7

where h is a thickness of said ZnO film, and λ is a wavelength of a surface acoustic wave.
 3. A surface acoustic wave device comprising:a substrate, an interdigital electrode on a surface of said substrate, and a piezoelectric film configured to cover said surface of said substrate and a surface of said interdigital electrode, wherein: said substrate is a langasite single crystal belonging to a point group 32, and when a cut angle of said substrate cut out of the langasite single crystal and a propagation direction of surface acoustic waves on said substrate are represented in terms of Euler angles (φ, θ, ψ), where 5°≦φ≦5° 85°≦θ≦95° -50°≦ψ<-35°, andsaid piezoelectric film is a c-axis oriented ZnO film that satisfies:

    h/λ=0.25 to 0.45

where h is a thickness of said ZnO film, and λ is a wavelength of a surface acoustic wave.
 4. A surface acoustic wave device comprising:a substrate, an interdigital electrode on a surface of said substrate, and a piezoelectric film configured to cover said surface of said substrate and a surface of said interdigital electrode, wherein: said substrate is a langasite single crystal belonging to a point group 32, and when a cut angle of said substrate cut out of the langasite single crystal and a propagation direction of surface acoustic waves on said substrate are represented in terms of Euler angles (φ, θ, ψ), where -°≦φ≦ ° 85°≦θ≦95° 35°≦ψ<-25°, andsaid piezoelectric film is a c-axis oriented ZnO film that satisfies:

    0<h/λ≦0.5

where h is a thickness of said ZnO film, and λ is a wavelength of a surface acoustic wave.
 5. A surface acoustic wave device comprising:a substrate, an interdigital electrode on a surface of said substrate, and a piezoelectric film configured to cover said surface of said substrate and a surface of said interdigital electrode, wherein: said substrate is a langasite single crystal belonging to a point group 32, and when a cut angle of said substrate cut out of the langasite single crystal and a propagation direction of surface acoustic waves on said substrate are represented in terms of Euler angles (φ, θ, ψ), where -5°≦φ≦5° 85°≦θ≦95° -25°≦ψ≦-10°, andsaid piezoelectric film is a c-axis oriented ZnO film that satisfies:

    0<h/λ≦0.45

where h is a thickness of said ZnO film, and λ is a wavelength of a surface acoustic wave.
 6. A surface acoustic wave device comprising:a substrate, an interdigital electrode on a surface of said substrate, and a piezoelectric film configured to cover said surface of said substrate and a surface of said interdigital electrode, wherein: said substrate is a langasite single crystal belonging to a point group 32, and when a cut angle of said substrate cut out of the langasite single crystal and a propagation direction of surface acoustic waves on said substrate are represented in terms of Euler angles (φ, θ, ψ), where -5°≦φ≦5° 85°≦θ≦95° 10°≦ψ<25°, andsaid piezoelectric film is a c-axis oriented ZnO film that satisfies:

    0<h/λ≦0.4

where h is a thickness of said ZnO film, and λ is a wavelength of a surface acoustic wave.
 7. A surface acoustic wave device comprising:a substrate, an interdigital electrode on a surface of said substrate, and a piezoelectric film configured to cover said surface of said substrate and a surface of said interdigital electrode, wherein: said substrate is a langasite single crystal belonging to a point group 32, and when a cut angle of said substrate cut out of the langasite single crystal and a propagation direction of surface acoustic waves on said substrate are represented in terms of Euler angles (φ, θ, ψ), where -°≦φ≦ ° 85°≦θ≦95° 25°≦ψ<35°, andsaid piezoelectric film is a c-axis oriented ZnO film that satisfies:

    0<h/λ≦0.45

where h is a thickness of said ZnO film, and λ is a wavelength of a surface acoustic wave.
 8. A surface acoustic wave device comprising:a substrate, an interdigital electrode on a surface of said substrate, and a piezoelectric film configured to cover said surface of said substrate and a surface of said interdigital electrode, wherein: said substrate is a langasite single crystal belonging to a point group 32, and when a cut angle of said substrate cut out of the langasite single crystal and a propagation direction of surface acoustic waves on said substrate are represented in terms of Euler angles (φ, θ, ψ), where -5°≦φ≦5° 85°≦θ≦95° 35°≦ψ<50°, andsaid piezoelectric film is a c-axis oriented ZnO film that satisfies:

    0<h/λ≦0.4

where h is a thickness of said ZnO film, and λ is a wavelength of a surface acoustic wave.
 9. A surface acoustic wave device comprising:a substrate, an interdigital electrode on a surface of said substrate, and a piezoelectric film configured to cover said surface of said substrate and a surface of said interdigital electrode, wherein: said substrate is a langasite single crystal belonging to a point group 32, and when a cut angle of said substrate cut out of the langasite single crystal and a propagation direction of surface acoustic waves on said substrate are represented in terms of Euler angles (φ, θ, ψ), where -50°≦φ≦5° 85°≦θ≦95° 50°≦ψ<70°, andsaid piezoelectric film is a c-axis oriented ZnO film that satisfies:

    h/λ=0.15 to 0.7

where h is a thickness of said ZnO film, and λ is a wavelength of a surface acoustic wave.
 10. A surface acoustic wave device comprising:a substrate, an interdigital electrode on a surface of said substrate, and a piezoelectric film configured to cover said surface of said substrate and a surface of said interdigital electrode, wherein: said substrate is a langasite single crystal belonging to a point group 32, and when a cut angle of said substrate cut out of the langasite single crystal and a propagation direction of surface acoustic waves on said substrate are represented in terms of Euler angles (φ, θ, ψ), where -5°≦φ≦5° 85°≦θ≦95° 70°≦ψ<90°, andsaid piezoelectric film is a c-axis oriented ZnO film that satisfies:

    h/λ=0.15 to 0.18

where h is a thickness of said ZnO film, and λ is a wavelength of a surface acoustic wave.
 11. A surface acoustic wave device comprising:a substrate, a piezoelectric film on a surface of said substrate, and an interdigital electrode on a surface of said piezoelectric film, wherein: said substrate is a langasite single crystal belonging to a point group 32, and when a cut angle of said substrate cut out of the langasite single crystal and a propagation direction of surface acoustic waves on said substrate are represented in terms of Euler angles (φ, θ, ψ), where -°≦φ≦ ° 85°≦θ≦95° -90°≦ψ<-70°, andsaid piezoelectric film is a c-axis oriented ZnO film that satisfies:

    h/λ=0.05 to 0.8

where h is a thickness of said ZnO film, and λ is a wavelength of a surface acoustic wave.
 12. A surface acoustic wave device comprising:a substrate, a piezoelectric film on a surface of said substrate, and an interdigital electrode on a surface of said piezoelectric film, wherein: said substrate is a langasite single crystal belonging to a point group 32, and when a cut angle of said substrate cut out of the langasite single crystal and a propagation direction of surface acoustic waves on said substrate are represented in terms of Euler angles (φ, θ, ψ), where -5°≦φ≦5° 85°≦θ≦95° -70°≦ψ<-50°, andsaid piezoelectric film is a c-axis oriented ZnO film that satisfies:

    h/λ=0.05 to 0.75

where h is a thickness of said ZnO film, and λ is a wavelength of a surface acoustic wave.
 13. A surface acoustic wave device comprising:a substrate, a piezoelectric film on a surface of said substrate, and an interdigital electrode on a surface of said piezoelectric film, wherein: said substrate is a langasite single crystal belonging to a point group 32, and when a cut angle of said substrate cut out of the langasite single crystal and a propagation direction of surface acoustic waves on said substrate are represented in terms of Euler angles (φ, θ, ψ), where -5°≦φ≦5° 85°≦θ≦95° -50°≦ψ<-35°, andsaid piezoelectric film is a c-axis oriented ZnO film that satisfies:

    0<h/λ≦0.45

where h is a thickness of said ZnO film, and λ is a wavelength of a surface acoustic wave.
 14. A surface acoustic wave device comprising:a substrate, a piezoelectric film on a surface of said substrate, and an interdigital electrode on a surface of said piezoelectric film, wherein: said substrate is a langasite single crystal belonging to a point group 32, and when a cut angle of said substrate cut out of the langasite single crystal and a propagation direction of surface acoustic waves on said substrate are represented in terms of Euler angles (φ, θ, ψ) where -°≦φ≦ ° 85°≦θ≦95° -35°≦ψ<-35°, andsaid piezoelectric film is a c-axis oriented ZnO film that satisfies:

    0<h/λ≦0.5

where h is a thickness of said ZnO film, and λ is wavelength of a surface acoustic wave.
 15. A surface acoustic wave device comprising:a substrate, a piezoelectric film on a surface of said substrate, and an interdigital electrode on a surface of said piezoelectric film, wherein: said substrate is a langasite single crystal belonging to a point group 32, and when a cut angle of said substrate cut out of the langasite single crystal and a propagation direction of surface acoustic waves on said substrate are represented in terms of Euler angles (φ, θ, ψ), where -5°≦φ≦5° 85°≦θ≦95° -25°≦ψ<-10°, andsaid piezoelectric film is a c-axis oriented ZnO film that satisfies:

    0<h/λ≦0.45

where h is a thickness of said ZnO film, and λ is a wavelength of a surface acoustic wave.
 16. A surface acoustic wave device comprising:a substrate, a piezoelectric film on a surface of said substrate, and an interdigital electrode on a surface of said piezoelectric film, wherein: said substrate is a langasite single crystal belonging to a point group 32, and when a cut angle of said substrate cut out of the langasite single crystal and a propagation direction of surface acoustic waves on said substrate are represented in terms of Euler angles (φ, θ, ψ), where -5°≦φ≦5° 85°≦θ≦95° 10°≦ψ<25°, andsaid piezoelectric film is a c-axis oriented ZnO film that satisfies:

    0<h/λ≦0.4

where h is a thickness of said ZnO film, and λ is a wavelength of a surface acoustic wave.
 17. A surface acoustic wave device comprising:a substrate, a piezoelectric film on a surface of said substrate, and an interdigital electrode on a surface of said piezoelectric film, wherein: said substrate is a langasite single crystal belonging to a point group 32, and when a cut angle of said substrate cut out of the langasite single crystal and a propagation direction of surface acoustic waves on said substrate are represented in terms of Euler angles (φ, θ, ψ), where -°≦φ≦ ° 85°≦θ≦95° 25°≦ψ<35°, andsaid piezoelectric film is a c-axis oriented ZnO film that satisfies:

    0<h/λ≦0.45

where h is a thickness of said ZnO film, and λ is a wavelength of a surface acoustic wave.
 18. A surface acoustic wave device comprising:a substrate, a piezoelectric film on a surface of said substrate, and an interdigital electrode on a surface of said piezoelectric film, wherein: said substrate is a langasite single crystal belonging to a point group 32, and when a cut angle of said substrate cut out of the langasite single crystal and a propagation direction of surface acoustic waves on said substrate are represented in terms of Euler angles (φ, θ, ψ), where -5°≦φ≦5° 85°≦θ≦95° 35°≦ψ<35°, andsaid piezoelectric film is a c-axis oriented ZnO film that satisfies:

    0<h/λ≦0.4

where h is a thickness of said ZnO film, and λ is a wavelength of a surface acoustic wave.
 19. A surface acoustic wave device comprising:a substrate, a piezoelectric film on a surface of said substrate, and an interdigital electrode on a surface of said piezoelectric film, wherein: said substrate is a langasite single crystal belonging to a point group 32, and when a cut angle of said substrate cut out of the langasite single crystal and a propagation direction of surface acoustic waves on said substrate are represented in terms of Euler angles (φ, θ, ψ), where -5°≦φ≦5° 85°≦θ≦95° 50°≦ψ<70°, andsaid piezoelectric film is a c-axis oriented ZnO film that satisfies:

    h/λ=0.05 to 0.7

where h is a thickness of said ZnO film, and λ is a wavelength of a surface acoustic wave.
 20. A surface acoustic wave device comprising:a substrate, a piezoelectric film on a surface of said substrate, and an interdigital electrode on a surface of said piezoelectric film, wherein: said substrate is a langasite single crystal belonging to a point group 32, and when a cut angle of said substrate cut out of the langasite single crystal and a propagation direction of surface acoustic waves on said substrate are represented in terms of Euler angles (φ, θ, ψ), where -°≦φ≦ ° 85°≦θ≦95° 70°≦ψ<90°, andsaid piezoelectric film is a c-axis oriented ZnO film that satisfies:

    h/λ=0.05 to 0.8

where h is a thickness of said ZnO film, and λ is wavelength of a surface acoustic wave.
 21. A surface acoustic wave device comprising:a substrate, an interdigital electrode on a surface of said substrate, and a piezoelectric film configured to cover said surface of said substrate and a surface of said interdigital electrode and an opposite electrode film on a surface of said piezoelectric film, wherein: said substrate is a langasite single crystal belonging to a point group 32, and when a cut angle of said substrate cut out of the langasite single crystal and a propagation direction of surface acoustic waves on said substrate are represented in terms of Euler angles (φ, θ, ψ), where -5°≦φ≦5° 85°≦θ≦95° -90°≦ψ<-70°, andsaid piezoelectric film is a c-axis oriented ZnO film that satisfies:

    0<h/λ≦0.1 or 0.3≦h/λ≦0.8

where h is a thickness of said ZnO film, and λ is a wavelength of a surface acoustic wave.
 22. A surface acoustic wave device comprising:a substrate, an interdigital electrode on a surface of said substrate, and a piezoelectric film configured to cover said surface of said substrate and a surface of said interdigital electrode and an opposite electrode film on a surface of said piezoelectric film, wherein: said substrate is a langasite single crystal belonging to a point group 32, and when a cut angle of said substrate cut out of the langasite single crystal and a propagation direction of surface acoustic waves on said substrate are represented in terms of Euler angles (φ, θ, ψ), where -5°≦φ≦5° 85°≦θ≦95° -70°≦ψ<-50°, andsaid piezoelectric film is a c-axis oriented ZnO film that satisfies:

    0<h/λ≦0.1 or 0.35≦h/λ≦0.8

where h is a thickness of said ZnO film, and ψ is wavelength of a surface acoustic wave.
 23. A surface acoustic wave device comprisinga substrate, an interdigital electrode on a surface of said substrate, and a piezoelectric film configured to cover said surface of said substrate and a surface of said interdigital electrode and an opposite electrode film on a surface of said piezoelectric film, wherein: said substrate is a langasite single crystal belonging to a point group 32, and when a cut angle of said substrate cut out of the langasite single crystal and a propagation direction of surface acoustic waves on said substrate are represented in terms of Euler angles (φ, θ, ψ), where -5°≦φ≦5° 85°≦θ≦95° -50°≦ψ<-35°, andsaid piezoelectric film is a c-axis oriented ZnO film that satisfies:

    0<h/λ≦0.15 or 0.35≦h/λ≦0.5

where h is a thickness of said ZnO film, and λ is a wavelength of a surface acoustic wave.
 24. A surface acoustic wave device comprising:a substrate, an interdigital electrode on a surface of said substrate, and a piezoelectric film configured to cover said surface of said substrate and a surface of said interdigital electrode and an opposite electrode film on a surface of said piezoelectric film, wherein: said substrate is a langasite single crystal belonging to a point group 32, and when a cut angle of said substrate cut out of the langasite single crystal and a propagation direction of surface acoustic waves on said substrate are represented in terms of Euler angles (φ, θ, ψ), where -°≦φ≦ ° 85°≦θ≦95° -35°≦ψ<-25°, andsaid piezoelectric film is a c-axis oriented ZnO film that satisfies:

    0<h/λ≦0.15 or 0.3≦h/λ≦0.5

where h is a thickness of said ZnO film, and λ is a wavelength of a surface acoustic wave.
 25. A surface acoustic wave device comprising:a substrate, an interdigital electrode on a surface of said substrate, and a piezoelectric film configured to cover said surface of said substrate and a surface of said interdigital electrode and an opposite electrode film on a surface of said piezoelectric film, wherein: said substrate is a langasite single crystal belonging to a point group 32, and when a cut angle of said substrate cut out of the langasite single crystal and a propagation direction of surface acoustic waves on said substrate are represented in terms of Euler angles (φ, θ, ψ), where -5°≦φ≦5° 85°≦θ≦95° -25°≦ψ≦-10°, andsaid piezoelectric film is a c-axis oriented ZnO film that satisfies:

    0<h/λ≦0.15 or 0.3≦h/λ≦0.45

where h is a thickness of said ZnO film, and λ is a wavelength of a surface acoustic wave.
 26. A surface acoustic wave device comprising:a substrate, an interdigital electrode on a surface of said substrate, and a piezoelectric film configured to cover said surface of said substrate and a surface of said interdigital electrode and an opposite electrode film on a surface of said piezoelectric film, wherein: said substrate is a langasite single crystal belonging to a point group 32, and when a cut angle of said substrate cut out of the langasite single crystal and a propagation direction of surface acoustic waves on said substrate are represented in terms of Euler angles (φ, θ, ψ) where -5°≦φ≦5° 85°≦θ≦95° 10°≦ψ<25°, andsaid piezoelectric film is a c-axis oriented ZnO film that satisfies:

    0<h/λ≦0.45

where h is a thickness of said ZnO film, and λ is a wavelength of a surface acoustic wave.
 27. A surface acoustic wave device comprising:a substrate, an interdigital electrode on a surface of said substrate, and a piezoelectric film configured to cover said surface of said substrate and a surface of said interdigital electrode and an opposite electrode film on a surface of said piezoelectric film, wherein: said substrate is a langasite single crystal belonging to a point group 32, and when a cut angle of said substrate cut out of the langasite single crystal and a propagation direction of surface acoustic waves on said substrate are represented in terms of Euler angles (φ, θ, ψ), where -5°≦φ≦5° 85°≦θ≦95° 25°≦ψ<35°, andsaid piezoelectric film is a c-axis oriented ZnO film that satisfies:

    0<h/λ≦0.5

where h is a thickness of said ZnO film, and λ is a wavelength of a surface acoustic wave.
 28. A surface acoustic wave device comprising:a substrate, an interdigital electrode on a surface of said substrate, and a piezoelectric film configured to cover said surface of said substrate and a surface of said interdigital electrode and an opposite electrode film on a surface of said piezoelectric film, wherein: said substrate is a langasite single crystal belonging to a point group 32, and when a cut angle of said substrate cut out of the langasite single crystal and a propagation direction of surface acoustic waves on said substrate are represented in terms of Euler angles (φ, θ, ψ), where -°≦φ≦ ° 85°≦θ≦95° 35°≦ψ<50°, andsaid piezoelectric film is a c-axis oriented ZnO film that satisfies:

    0<h/λ≦0.45

where h is a thickness of said ZnO film, and λ is a wavelength of a surface acoustic wave.
 29. A surface acoustic wave device comprising:a substrate, an interdigital electrode on a surface of said substrate, and a piezoelectric film configured to cover said surface of said substrate and a surface of said interdigital electrode and an opposite electrode film on a surface of said piezoelectric film, wherein: said substrate is a langasite single crystal belonging to a point group 32, and when a cut angle of said substrate cut out of the langasite single crystal and a propagation direction of surface acoustic waves on said substrate are represented in terms of Euler angles (φ, θ, ψ), where -5°≦φ≦5° 85°≦θ≦95° 50°≦ψ<70°, andsaid piezoelectric film is a c-axis oriented ZnO film that satisfies:

    0<h/λ≦0.05 or 0.2<h/λ≦0.8

where h is a thickness of said ZnO film, and λ is a wavelength of a surface acoustic wave.
 30. A surface acoustic wave device comprising:a substrate, an interdigital electrode on a surface of said substrate, and a piezoelectric film configured to cover said surface of said substrate and a surface of said interdigital electrode and an opposite electrode film on a surface of said piezoelectric film, wherein: said substrate is a langasite single crystal belonging to a point group 32, and when a cut:angle of said substrate cut out of the langasite single crystal and a propagation direction of surface acoustic waves on said substrate are represented in terms of Euler angles (φ, θ, ψ), where -5°≦φ≦5° 85°≦θ≦95° 70°≦ψ<90°, andsaid piezoelectric film is a c-axis oriented ZnO film that satisfies:

    0<h/λ≦0.5 or 0.25≦h/λ≦0.8

where h is a thickness of said ZnO film, and λ is a wavelength of a surface acoustic wave.
 31. A surface acoustic wave device comprising:a substrate, an opposite electrode film on a surface of said substrate, and a piezoelectric film on said opposite electrode film and an interdigital electrode on a surface of said piezoelectric film, wherein: said substrate is a langasite single crystal belonging to a point group 32, and when a cut angle of said substrate cut out of the langasite single crystal and a propagation direction of surface acoustic waves on said substrate are represented in terms of Euler angles (φ, θ, ψ), where -5°≦φ≦5° 85°≦θ≦95° -90°≦ψ<-70°, andsaid piezoelectric film is a c-axis oriented ZnO film that satisfies:

    h/λ=0.05 to 0.8

where h is a thickness of said ZnO film, and λ is a wavelength of a surface acoustic wave.
 32. A surface acoustic wave device comprising:a substrate, an opposite electrode film on a surface of said substrate, and a piezoelectric film on said opposite electrode film and an interdigital electrode on a surface of said piezoelectric film, wherein: said substrate is a langasite single crystal belonging to a point group 32, and when a cut angle of said substrate cut out of the langasite single crystal and a propagation direction of surface acoustic waves on said substrate are represented in terms of Euler angles (φ, θ, ψ), where -°≦φ≦ ° 85°≦θ≦95° -70°≦ψ<-50°, andsaid piezoelectric film is a c-axis oriented ZnO film that satisfies:

    h/λ=0.05 to 0.8

where h is a thickness of said ZnO film, and λ is a wavelength of a surface acoustic wave.
 33. A surface acoustic wave device comprising:a substrate, an opposite electrode film on a surface of said substrate, and a piezoelectric film on said opposite electrode film and an interdigital electrode on a surface of said piezoelectric film, wherein: said substrate is a langasite single crystal belonging to a point group 32, and when a cut angle of said substrate cut out of the langasite single crystal and a propagation direction of surface acoustic waves on said substrate are represented in terms of Euler angles (φ, θ, ψ), where -5°≦φ≦5° 85°≦θ≦95° -50°≦ψ<-35°, andsaid piezoelectric film is a c-axis oriented ZnO film that satisfies:

    h/λ=0.05 to 0.45

where h is a thickness of said ZnO film, and λ is a wavelength of a surface acoustic wave.
 34. A surface acoustic wave device comprising:a substrate, an opposite electrode film on a surface of said substrate, and a piezoelectric film on said opposite electrode film and an interdigital electrode on a surface of said piezoelectric film, wherein: said substrate is a langasite single crystal belonging to a point group 32, and when a cut angle of said substrate cut out of the langasite single crystal and a propagation direction of surface acoustic waves on said substrate are represented in terms of Euler angles (φ, θ, ψ), where -5°≦φ≦5° 85°≦θ≦95° -35°≦ψ<-25°, andsaid piezoelectric film is a c-axis oriented ZnO film that satisfies:

    h/λ=0.05 to 0.5

where h is a thickness of said ZnO film, and λ is a wavelength of a surface acoustic wave.
 35. A surface acoustic wave device comprising:a substrate, an opposite electrode film on a surface of said substrate, and a piezoelectric film on said opposite electrode film and an interdigital electrode on a surface of said piezoelectric film, wherein: said substrate is a langasite single crystal belonging to a point group 32, and when a cut angle of said substrate cut out of the langasite single crystal and a propagation direction of surface acoustic waves on said substrate are represented in terms of Euler angles (φ, θ, ψ), where -5°≦φ≦5° 85°≦θ≦95° -25°≦ψ<-10°, andsaid piezoelectric film is a c-axis oriented ZnO film that satisfies:

    h/λ=0.05 to 0.45

where h is a thickness of said ZnO film, and λ is a wavelength of a surface acoustic wave.
 36. A surface acoustic wave device comprising:a substrate, an opposite electrode film on a surface of said substrate, and a piezoelectric film on said opposite electrode film and an interdigital electrode on a surface of said piezoelectric film, wherein: said substrate is a langasite single crystal belonging to a point group 32, and when a cut angle of said substrate cut out of the langasite single crystal and a propagation direction of surface acoustic waves on said substrate are represented in terms of Euler angles (φ, θ, where -°≦φ≦ ° 85°≦θ≦95° 10°≦ψ<25°, andsaid piezoelectric film is a c-axis oriented ZnO film that satisfies:

    h/λ=0.05 to 0.45

where h is a thickness of said ZnO film, and λ is a wavelength of a surface acoustic wave.
 37. A surface acoustic wave device comprising:a substrate, an opposite electrode film on a surface of said substrate, and a piezoelectric film on said opposite electrode film and an interdigital electrode on a surface of said piezoelectric film, wherein: said substrate is a langasite single crystal belonging to a point group 32, and when a cut angle of said substrate cut out of the langasite single crystal and a propagation direction of surface acoustic waves on said substrate are represented in terms of Euler angles (φ, θ, where -5°≦φ≦5° 85°≦θ≦95° 25°≦ψ<35°, andsaid piezoelectric film is a c-axis oriented ZnO film that satisfies:

    h/λ=0.05 to 0.5

where h is a thickness of said ZnO film, and λ is a wavelength of a surface acoustic wave.
 38. A surface acoustic wave device comprising:a substrate, an opposite electrode film on a surface of said substrate, and a piezoelectric film on said opposite electrode film and a n interdigital electrode on a surface of said piezoelectric film, where in: said substrate is a langasite single crystal belonging to a point group 32, and when a cut angle of said substrate cut out of the langasite single crystal and a propagation direction of surface acoustic waves on said substrate are represented in terms of Euler angles (φ, θ, ψ), where -5°≦φ≦5° 85°≦θ≦95° 35°≦ψ<50°, andsaid piezoelectric film is a c-axis oriented ZnO film that satisfies:

    h/λ=0.05 to 0.45

where h is a thickness of said ZnO film, and λ is a wavelength of a surface acoustic wave.
 39. A surface acoustic wave device comprising:a substrate, an opposite electrode film on a surface of said substrate, and a piezoelectric film on said opposite electrode film and an interdigital electrode on a surface of said piezoelectric film, wherein: said substrate is a langasite single crystal belonging to a point group 32, and when a cut angle of said substrate cut out of the langasite single crystal and a propagation direction of surface acoustic waves on said substrate are represented in terms of Euler angles (φ, θ, ψ), where -°≦φ≦ ° 85°≦θ≦95° 50°≦ψ<70°, andsaid piezoelectric film is a c-axis oriented ZnO film that satisfies:

    h/λ=0.05 to 0.8

where h is a thickness of said ZnO film, and λ is a wavelength of a surface acoustic wave.
 40. A surface acoustic wave device comprising:a substrate, an opposite electrode film on a surface of said substrate, and a piezoelectric film on said opposite electrode film and an interdigital electrode on a surface of said piezoelectric film, wherein: said substrate is a langasite single crystal belonging to a point group 32, and when a cut angle of said substrate cut out of the langasite single crystal and a propagation direction of surface acoustic waves on said substrate are represented in terms of Euler angles (φ, θ, ψ), where -5°≦φ≦5° 85°≦θ≦95° 70°≦ψ<90°, andsaid piezoelectric film is a c-axis oriented ZnO film that satisfies:

    h/λ=0.05 to 0.8

where h is a thickness of said ZnO film, and λ is a wavelength of a surface acoustic wave. 